Is it a bad idea to use a .biz domain? Somehow implying that you're selling something not getting them through to ebay? Also do most of you buy a separate domain for each website you make or do you buy one and make sub domains off of it?
.biz is cool - I have used .infos ($2.99 on godaddy) too, it may result in a minor loss (minor minor) but with ebay/cj (I assume you mean) the commisions are likely to be micro anyhow - Would be interested in a split test sitauation but personally see equal success with both .info's, .coms and .co.uks. You can do subdomains, if you get 2.99 .info's though its cheap enough to be cost efficient on even tiny returns.
the Search engines don't care. Well maybe .edu are treated with a bit more regard but I have .info .biz .net and .org that all do fine
I think it is tough for the general public to remember .biz, we here are all savvy internet people, but the general public really only remembers .com in my experience...