I buy from and sell to overseas eBayers and BF members frequently. It's not a big deal. The 1/2 hour line at the PO is true wherever you're trying to ship (as is the phenomenon that the longer the line, the fewer employees at the windows!). And as iab points out, the customs form, if you know you need to fill it out, is simple and easily done before you get to the USPS clerk. Even more interesting to me is that I often receive small parcels much more quickly from overseas (mostly western Europe) than I do from CONUS.
For an eBay auction, by opening up to more bidders, i.e., more competition, you increase the chances of the bid ending higher. That's certainly worth it in my view.
Neal