I realize some of these seem like silly questions and I'm sure I could find the answer to all of them if I searched hard enough, but I figured I should ask anyway.
1.) yes there is a search bar on the buy tab
2.) from what i've seen anything that is normally tradeable
I wish they would finish it before the release. Currently i don't see any way to trade one mount for another, for example, or to trade something for thrices or quads. It's really weird that you can trade for pentas, but not for quads. I also wonder how people on PTS are trading for golden souls - that was the primary currency there.
Still huge step forward though, long awaited and much desired feature.