If you're nervous break up the $500 into smaller amounts to reduce the loss if scammed. So send $100 and after a couple days after they sent the coins, send the next hundred.
Or if you really want to speed up the process, send $100 to 5 different exchangers all at once.
Or, you know, don't send straight cash in the mail to anyone at all.
You guys are getting way too conditioned to getting scammed if this plan is at all acceptable.