Practically any GPS will do this, so you could even pick up a simple older unit for $50 or so, and not ruin your geocaching experience one bit.

That's true. One note though. The 8-channel Garmin 75 that I bought for a trip in 1993 ($575) still works fine (out on open water...) It's my sense, though, that the little 12-channel Garmin eTrex I bought last year on sale for $99 definitely outperforms the old unit in irregular terrain, under some forest canopy, et cetera.

The interface on the eTrex is pretty dumbed down, and I wish it had a numeric keypad like the old 75, but it's great for travel. Anyhow, my bottom line would be that, if you go the eBay route, I wouldn't get anything *too* old when there are fairly cheap newer alternatives that might serve you just a bit better in them thar geocaching hills.