I've bought several IDE drives via eBay and they've all worked fine on my A1200/IDE adapter. I think what you must watch out for is bidding on second hand (or even new) drives and going well beyond the price+delivery you'd pay to a commercial supplier advertising in computer magazines (eg MicroMart).
It's been discussed around here several times, but people bidding on eBay often get carried away trying to win an item they could order directly from a commercial website at a cheaper cost. If you enjoy the challenge of eBay, go ahead and have fun, but set yourself a budget price for anything as common as a hard drive. I've just won a second hand Samsung 40Gb drive and now see a brand new 40Gb Maxtor advertised by a major supplier for five pounds less.
Cheers,
JaX
