In the past... maybe a decade since I got the last batch. First lot years back was a bunch of rando socket 7 mostly, I think I got most of them working. Then next lot was a bunch of P2B series boards, "for parts" and they were in a poor state had components robbed out already, I was hoping to piece together a couple from it, but all I got working was a P2L-97 that was in with them. However, it was maybe not my first, but an early rodeo with slot 1, so when the box appears again, I'm gonna throw the book of heavy knowledge subsequently mentally accumulated at them and see if more resurrections can happen. Then the most recent lot probably around 2009 was a very mixed bunch of socket A, socket 370 and a couple of slot 1s, half of them had bloated caps, probably up to about 80% working out of that batch now, as I just brung up two of the 370s that I couldn't get much out of before. Also I don't know if the remaining few boards with bad caps might work if recapped, didn't get round to them as they weren't too exciting, SDRAM socket As. Think they were sold as scrap lot. Oh yah, now I think of it, I probably got a lot of 486 boards around 99-00 which had a high proportion working.
Mostly, these were opportunistic buys at prices about the level of "one good guaranteed to work" board price, plus the shipping being about double. Now if I were looking for lots, I'd try for ones with a particular "smell" i.e. clearing out the IT closet, forgotten takeouts from a mom and pop computer store, what you don't want to be is the schmuck who is making the third or fourth attempt at getting them working since they were uninstalled... all the easy pickings have been picked. Unless of course you have Louis Rossmann level rework skills and all the equipment to do chipset level diagnosis... and don't mind spending the time doing it. If you don't want to even solder capacitors, or knocked off components or scratched traces, then this is probably not for you. However, if you're the first to have a go at an unmolested batch, you may find you get up to 30% or so working just from paying attention to configuration and CPUs board is guaranteed compatible with (i.e. lowest gen it supports, so you can flash BIOS for later gen, or are familiar with hotflashing or own a ROM burner) But that won't hold true if you can see in the pics that every single cap is bloated of course.
But yeah, it's a whole passel of Caveat Emptor and they usually only carry the 50/50/50 guarantee.... in 50 mins they will be 50 miles away with $50 of your hard earned spondulix, so good luck with that.
Unicorn herding operations are proceeding, but all the totes of hens teeth and barrels of rocking horse poop give them plenty of hiding spots.