I've only done a few trades, so I'm not exactly a seasoned pro, but I'll give you my 2-cents.
First of all, trading is a lot safer than buying, in my opinion, since scalpers and cheats are less motivated to trade than sell.
Trading by mail is fine, just be sure to send tickets by registered mail.
I've traded ticketfast tickets, but you really need to trust each other and I wouldn't do that with someone that didn't buy the ticket direct from TM.
For me it comes down to sussing out whether you're dealing with a genuine U2 fan or not (which admittedly isn't that easy by email). Throw in a friendly comment or question about the band or tour and see how they respond. . .if your gut tells you your dealing with a fan, relax. . .U2 fans are highly honorable in my experience and eager to help each other out.
If you're not sure, or your correspondent is strictly business, you might think twice.
In any case, take your time and don't act out of desperation. If it doesn't feel right, wait for another opportunity.
Just one fan's opinion. . .