1. I would recommend it. Make sure you aren't getting ripped off...you should probably get more than Mayhem is worth depending on how many cards you're getting out of the deal, and what they are. If you're trading for tin/TexP/Di cards, yeah, you better be getting a lot of stuff. If it's just ultras, it would probably be pretty close, if the trade involved KoT or Lampy, etc.
2. Yeah. Especially with promos, as they generally drop value pretty quickly. It does depend on the person though. I like to switch decks a lot, so I need a little bit of everything so I can use whatever I want.
3. High end cards, useful cards, and a list of things you need.