Well, I decided to bite the bullet and roll the dice (and mix the metaphors)....
I have bought a large lot of Magic cards - 300+ - for $23 total.
The lot was up for auction with less than an hour left. The person seems to not know completely what they have (it seems they, like me, were into Magic years ago and, based on their description they have no clue about the more recent sets). So, there may be gems in there. Hopefully so. The lot had been bid up to $13.50, with shipping at $7.50. I figured I was willing to go to $30 total, or ~$0.10 per card. So, I waited until there was a minute left, bid $22.50 and waited. I won it for $15.50. Woo-hoo!
Now I wait for the cards to arrive and see if I came up with a royal flush or a lousy pair of twos (more metaphors! Yippee! Are you keeping count?).
I figure even if I come out with 300+ commons, the worst I'm out is ~$13 bucks, as I ought to be able to sell them for $0.05 each or so. But I'm not TOO concerned - this person sold a 400+ card lot a little bit ago and the person who left feedback seemed pleased with what they got.
If this turns out well, I will probably look into doing more of this. This is how you make money on Magic on eBay - you buy large lots from people who don't know what they have, or who are just trying to dump their cards, then pick the lots apart and sell the cards individually.
I will, of course, keep you posted here on the results!