You can just exchange cash in envelopes. jk
What I was going to say. Send birthday card or something with money in it.
I've actually done that a couple time (for small cost items), no problem. For the most part, people with history/reputation here aren't looking to rip you off.
There's also the fallback of public humiliation, and of course, you have their home or work address. And as everybody knows, people from the internet are crazy, so you shouldn't rip them off when they know where you live.