Coming from having done online trading for a good duration of time with D&D Minis over at maxminis (not even sure if it survived after the database issue or not, but whatever), I feel that sending first is a good way to go about things.
Why?
Because people who are "unestablished" are more likely to cause issues or result in a bad trade, especially when they can just claim they're trading something but do not actually have it.
I have never had a bad trade there, and so far things are going well here. If you communicate well, one should not feel insulted just because they're asked to send first when someone has several trades.
In some instances, people have just sent to me even though I have said I could not or will not send until a certain date. Were they worried? Probably a bit, but it's nice to know that people will send.
I, like others on here I'm sure, want to get my cards as fast as possible since I'm trading to be able to get cards I want to use immediately (or sometimes for other trades). If I send and they send same day, then we both get our cards faster and are happier.
End result, if asked to send first, it's more a security thing--and could still result in a bad trade, but it is less likely. Do not feel insulted. If you don't want to send first, then don't do the trade; no one is forcing you. Just communicate and everything will flow nicer for yourself and the people you trade with.