I've done this once. Really I think it boils down to the integrity of the person you are trading with, you really want to make sure you've gotten a sense of security. Talk to the guy on the phone, ask for references in other online sales/trades (say for example Ebay account feedback, or dealings on other forums).
You also want to give him some assurance, so give him references, be open to contact and questions.
Once it comes down to finally making the trade, carefully document it all, the terms, save all emails, personal info, serial numbers of the guitars, and take some pictures incase any goes wrong. Exchange receipts and tracking info for the guitars. I like shipping through the UPS stores because they will package for you for a fee, but that also insures that if for any reason it is damaged due to packaging it is UPS that will cover it.
Hope this helps.