I've had some good luck on Ebay. Not necessarily cards, but I did have a few good buys on odd listings.
Pretty beat George C Miller that was listed as "old baseball card" And described as "maybe from the 30's?" Yes, that was the whole description. And I was the only bidder
Recently, a T206 listed as trimmed that almost certainly isn't. It's short, but the edges seem factory, and it's VG with no odd wear, just typically rounded corners.
Now with bicycles and stamps I've done very well spotting stuff that's been listed very poorly, often as a BIN.
A batch of bike parts that showed three parts, but said in the listing pumphead(235)each dropout set(50) each etc, So I asked if it was just what was shown, or if it was actually 235 of one part and 50 of the other. The answer - Yes, 235 of one, 50 of another, plus some other stuff and a few partial dropout sets. All for I think 250 BIN and 25 shipping. About 3-4K retail.
The stuff I've picked up for my collection has been more fun though. And I'm often the only bidder.
Steve B