I have. One lot I bought had like 30 motherboards plus a bunch of other stuff.

They were all older boards for the most part. If they are newer boards being sold like that they probably aren't worth the trouble.

That being said, the eBay market for good vintage "scrap lots" has pretty much dried up and the scrap lots being sold now are mostly junk and the sellers think they can get outrageous prices for a bunch of physically broken junk.

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.

Not for a long time... the last one I bought like three or four years ago on eBay and only about 10% of them actually powered on. Some had further issues. It wasn't a good deal in the end 🤣

This makes me wonder whether the hoarding behaviors have changed over time.

In the mid-1990s I had to throw away a few large boxes full of working Apple II (+clones) boards, 8088 and 80286 boards and cards that piled up over time.
Didn't stash away/hoard boards that were not fully functional, these went directly to the trash.
In the mid 2000s I threw another big batch of obsolete working stuff, 386, 486 boards, lots of ISA+VESA cards, perfectly good MFM and RLL drives and even an early PET-2001 mobo to the trash.

Last time I threw away retro stuff was 2018, a big box of good RAMs and CPUs of all ages, shortly before thre retro virus caught me. Argh.

Nowadays the collections seem to consist mainly of non-working/defective stuff. Why, I ask myself.

IMO if things go dead or need extra attention (looking up right settings for CPU replacement etc) while they are current enough to be useful still if you get them working, then they get stashed somewhere to look at or fix "sometime" then get forgotten about. If they are substantially obsolete when this happens they get chucked. If they are working great and just a little over the hill they got given away or sold working.

So, I have a theory that the capacitor plague saved more boards long term than it killed short term. Boards died while they were not yet obselete enough not to care, got saved for the "sometime" fix. The plague in general normalised capacitor replacement, some of the boards recapped then with superior quality caps may have had longer working lives than "pre plague" boards that just had "okay" caps. So even after plague era, the experience and knowledge is there in cap replacement and boards wearing out their cheap or okay caps in due time have had the opportunity to get re-capped if still useful, or sought after due to being a popular board of a more recent era.

on the whole it sounds from your experiences that this was something worth considering in the past but is increasingly not worth doing now

a working rate of as low as 10% is poor and i'd agree with the general principle that you buy for just a shade over the price you'd pay for 'one good board'

interest capacitor plague theory though BitWrangler, inadvertently saving boards by being shelved as 'fix later', I've been reasonably fortunate so far in terms of seeing few bad caps

it's still temping, especially knowing how robust some boards are - even working when looking awful

BTW I'd definitely recommend having a POST card, I managed years without one, mostly due to thinking the pricing was scammy when they were several hundred bucks with like 3 chips on, but they're much cheaper and easier to get these days. Anyway, they really help in focusing attention on which part of the board is misbehaving. This is very often a quite minor thing, this last year, mine pointed me to a dirty CPU socket, an i/o chip with a bad connection, an incompatible vid card, a memory problem that was a tad more subtle than causes the no-ram bleeps. All these were quite quickly found and fixed, but while appearing minor, they were causing a no boot condition where otherwise you'd be at a "check everything" point, and you could miss stuff, like the CPU socket had already had a wipe, the vid and ram worked fine in other systems etc etc. One board had multiple simple problems and in an hour or so I was able to knock them out one by one and get it booting.

edit: you do have to Sherlock Holmes it a bit though, it's like OBD-II codes, an Oxygen sensor code does not necessarily mean you need a new O2 sensor, first you've got to check if you've got a hole in the manifold (I mean it should be loud, but you'd be amazed the ppl who think it's unrelated and they can fix that later) or that you don't have an injector plugged up and one cylinder pumping fresh air (It's running a bit lumpy but if I swap the O2 out it's bound to be okay...)