Why is this such a big deal? If you do something terrible, the other guy will leave you feedback saying as much. (I would, anyway). When everything goes fine, a lot of the time you forget. Like people have said, those that really care about building up their trust will email you about it.
I've probably been part of about 50 trades, and have about half that much feedback. Anything above 10 should be enough to convince people you're legit, so surely not a big problem? Just send polite reminders to people you trade with until you have enough feedback to be happy...