The 1980s were when card production really went freaking insane culminating in the junk wax era of 1986/7 to 1991/2 or so. However, a few things are true
- There are still valuable cards in this era - but they are almost all rookie cards, short printed subsets, or weird errors
- The "vibes" of the era do permeate to earlier and later years but are less true as you get further
- What you'll be able to get out of it will directly correlate to the amount of work you put into it.
I'd argue anything before 1986 or so is not junk era but the lead in to junk era - and anything after 1992 is when you really start entering the Insert/90s Era.
So first, if you have any cards from that 87-92 period - all I'd really bother doing is to pull out any notable rookies. That's gonna represent most of what matters in those sets and even what matters is minimal. Only a handful of those are anything more than dollar bin fodder. The further back you go from there the wider net you can cast. Like, take 1984 Topps...the most valuable card by far in that set is the Don Mattingly RC. But there's also Darryl, second year (?) Gwynn, and other players worth pulling and holding. You get back to like 1981 has Fernando and Kirk Gibson - not HoF types but still can carry some value in good condition. Additionally, once you get out of junk era, you have collectors who really begin to value high conditioned cards - and the condition will matter a ton to buyers.
I think if you told a card shop that you had cards from the "late 70s to the late 80s" and they outright declined to even look they're fools or assumed you meant a tow truck full of 1988 Topps.
So like others have said, if he is willing to take some time, he should pull out Hall of Famers, Hall of Very Gooders, Rookies of Notable Names, and then start evaluating for condition and price. It'll be like panning for gold but there is some gold that exists. Personally, I'd look through this blog - which has card photos, descriptions, etc. of the most prominent cards from a ton of major sets to get an idea what to pull - ignore the prices because that's PSA10 and pack pulled cards don't usually get 10s - but it should help you identify what matters - I'll link to '85 Topps but there's a lot in this blog
25 Most Valuable 1985 Topps Baseball Cards - Old Sports Cards
Other than that though, it's all bulk. I wouldn't bother trying to move most of it - I've been at estate auctions where there are literally massive totes of these cards in sorter boxes where 30k+ cards go for $15 if anyone is willing to haul them out. Put them out at a yard sale with a $5 price tag if you like.
Now, for modern, it's a very different ballgame - though bulk still verges on worthless it's less print run and more that set building is a dying/dead art with eBay and breakers out there. A quick rundown of your most popular sales methods
Card Shows: Yep, these still exist. A bunch of folding tables in an Elks Lodge with a mix of (mostly) men with tables of overpriced cardboard and uncomfortably small aisles. However, I think they are a great learning/entrepreneurial experience. Basically you (as a vendor) rent table space for a day, set up your cards, and work out a deal or a ton. I'd generally recommend you have a good amount of cards to sell - I'd say at least 50-100 "displayable" cards and ideally a nice sorting box or two of cheap singles, make sure you have the ability to take at least cash and Venmo/Paypal - it's a great quick lesson in deal making, preparedness, and nice way to convert cards into money or other cards without a ton of packaging/shipping. I'd recommend it as a parent-child thjng for sure as it's definitely easier with two people to start. I can get into details on this if you want - never sold but I've seen good and bad tables and I know what I like. High physical effort/work component but a good way to keep most of your cash with volume and trade. The toughest part of this minus the physical mechanics is really just doing the prep to know your card prices to avoid getting ripped off or scaring away buyers, and having the backbone/knowledge on when to say no, yes, or maybe.
Edit: I've noticed a lot of tables generally have a specialty or area they focus in - you'll often see single-sport tables, non-sports/wrestling tables, vintage tables, ultra-modern tables. You don't need to specialize that much but it may help if he has a lot to move to do so just to let customers self-select and dig.
Card Shops / Trade Nights : A lot of local card shops (LCS) will buy cards and/or host trade nights. LCSes will buy "liquid" cards (easy to move) at a discount - but can be an easy way to dump cards. Trade Nights can be great for converting cards you don't want to ones you want and working on negotiation skills, meeting collectors, etc. I am sure each will have its own rules around including cash or not
Consignment : Some card shops and antique/flea settings will let you consign cards - so basically you give them to the store who will price (or you price) and if they sell it, you each get a share. More commonly today, there's also online Consigners where you send incards, they take their fees, and you use their platform to list
and ship cards. I consign the bulk of my cheaper singles I don't want to bother prepping for individual shipping with COMC but there's a ton out there - especially if you decide to consign more expensive cards. If he does want to go this way let me know and I can give a lot more on this - even as someone who doesn't sell a ton I've probably consigned and sold about $1k in cards on COMC and got a shipment of 900 or so going out this week to them.
Online Auctions (Ebay) - Ebay is the undisputed king of sports card sales. Just a complete stranglehold on all but the super high end market. Basically, you use their platform to post your card for auction/sale - it sells, you get payment to your account, you ship the card to the address they give. Ebay deserves its own book but if he's really gonna get into selling a lot of volume it's worth getting used to eBay
Online Forums (Reddit, Blowout Forums, etc.): The "bridge" between card shows and online auctions - you can sell on Reddit, Facebook, Instagram, Twitter and other forums. Same overall structural elements, with some more manual work, but it's a good training ground. I did my first real person-to-person sales via Reddit and got some legitimately great advice and guidance from people who understood this was newer for me.
Online Marketplaces (Whatnot): The newest entrant with notable cache is Whatnot. Honestly at this point it feels really scummy, like modern day self-run Shop-At-Home - but you can list and sell singles on there by running a video or even just in a marketplace setting. I consider this a more advanced sales tool designed for breaks more than card sales but it's not the craziest way to do this either.
My personal recommendation if he's looking to sell would be to do some small test runs. It gets easier every time you do it but there's a LOT of nuance to selling cards well online in terms of material, packing, timing, etc. and card collectors are often kinda dickish. I like the card show idea if he has a reasonable quantity he wants to sell - I usually see a few parent-child tables any time I go and everyone gets into it - and people will probably be less forward if a younger seller is working it. Finally, it's worth noting that card sales like anything else is rooted in an "it takes money to make money" angle. Any of these methods will require an upfront investment in material and time to earn your money back and will reward the hard work - and it'll be difficult to start. The boom of sports cards with all the scammers, hype men, etc. it brought in made it look way easier than it is and the people really good at this are INSANELY good at what they do from product/card selection, timing on selling, knowing when to negotiate/let go, etc.
Finally, if you are looking for any personalized advice or thoughts, please feel free to reach out. I find a lot of people in this hobby are genuinely supreme humans...and a lot aren't.