I buy them both because I like them but also because I can't be bothered to try to beat all the sharks and resellers to the car boot and stick my head in cars as people are setting out to buy console games (got to one early the other day and went in reverse only to meet the parade of kids arbitraging things on their phones with amazon, ebay, cex and the like). PC games are a bit more sedate and not as sought after, aside from the exception for the nice cardboard boxed PC games which is a whole world unto itself.
To that end I can still get some interesting stuff in car boot sales, yard sales, garage sales, flea markets, charity shops and the like. Even grandma knows to check ebay for NES games these days (another thing that reduces the availability compared to 10-15 years ago, which was still firmly in this retro games is cool boom) but PC games still seem to be lumped in with worthless DVDs and CDs, especially the best of/second run sets (or bundles) that would occupy the spinning racks/towers in game shops and general goods shops a year or two after the initial release of a game. If physical trinkets are your thing then maybe not the life for you but if games (and usually all patched up* and with any expansion packs (that is what DLC was called back when there kids in the audience). I was in the US (western Washington state too so fertile ground for competition from nerds) within the last year as well and could have done pretty well in the various thrift shops, game shops (fewer of them than previous visits and not many into PC stuff compared to overpriced console games), yard sales and antiques shops (people rent cabinets from the owners in most places so I do see a lot of games as the price density and presumably sales volume of such things compared to trinkets grandma likes is worth it for some) I wandered through when there.
Other than that it is the same as everything else -- your target is bored mothers, grandmothers and spurned lovers selling things from the house via whatever means (facebook groups for locals, craigslist, gumtree... you might have to buy bundles to keep what you want and cycle on the rest, you have to ask people when buying things if they have old PC games as most won't associate it with them. Some also put a word in with people doing house clearance to give them first dibs on PC games (again you might have to buy volumes to make it worth their while) -- a 30 something neckbeard in the 80s-90s is almost prime heart attack and dying alone territory right now (next 10-15 years will be a golden age for that).
*few games even reserved patches for the expansion only or gold versions set. Turned down a copy of Dungeon Lords the other day (game I quite liked, had LAN co-op on PC back when which is a bit of a rarity. Nice worldbuilding as well) for that reason.
The PC boxed games world is also tempered a bit -- the death of the physical PC game is going to be a hotly debated topic in history in the years to come but I will put at least a major inflection point at around the time of the orange box and Valve's/Steam's ascendancy. After that we also started to see major phone home checks and physical games that were a steam installer and a key.
It also goes a bit retro and many devs or pubs realised the demand and on services like steam, good old games, the pub's own service they are trying to push and the like so might well either issue some fixes, bundle dosbox or otherwise sell a copy of the game online which many have even confessed to buying rather than sorting out a DVD drive or floppy drive for their laptop or something.
Some might also ponder piracy as it was back then -- between compression being a viable thing (once saw a later entry in the worms series crushed to something like 40 megabytes from CD or so size), no CD patches, generous return policies of game shops and the rise of p2p (torrents towards the end, said worms was from a sort by size on probably shareaza) then only the music tracks being 3 megs a piece for your MP3s is ever likely to have beaten it (maybe books today).
Choice video on boxes.
You also have the question of what to run it on. It seems retro is a thing in the PC world nowadays and even a basic 486 (never mind some kind of decent mobo with a DX 100 and maxxed out RAM) will set you back as much as a decent modern refurb machine. Want a high end original soundblaster (as opposed to clone, though modern FPGA clones are becoming a thing) or a bit later a nice 3dfx graphics card* and the prices will be as eye watering as they were back when they were current.
Personally I have yet to find a combo of source ports
https://osgameclones.com/ , modern patches
https://www.wsgf.org/mgl , my generally decent but aged hardware (
https://gbatemp.net/threads/new-pc-time.42858/ is kind of around still , core2 stuff straddles the line between old and new worlds well enough), virtual machines (
https://www.virtualbox.org/ ), various emulation options (
https://www.pcem-emulator.co.uk/ for an interesting twist, 3dfx emulators go back a very long time as well) or dosbox (
https://www.dosbox.com/ ) that does not do it for me so I go with that. Others need that CRT glow, 30 year old fire retardant smell and beige plastic.
*there are also oddities like the last of a given slot type being worth a fortune despite being considered a joke at the time. For instance around the later directX9 era also came the end of AGP graphics slots. The AGP version of this graphics card got about half the performance of the PCIE version as far as polygons pushed but today is worth a fortune as it the best choice you have for graphics on an AGP board, where the PCIE version is maybe not quite computer scrap (said era is now long enough ago to be a type of retro) but way way less.
