Well then, If there is a tracking number I believe you can call USPS or UPS or Fedex and tell them to return to sender at whatever point it is in the shipping process (that an error has occured and it will need to be returned to sender)...complicated, a pain, yes but it is a solution. Too bad this mess happened, it seems everybody gave it a good chance to resolve...tarptents do come up quite regularly on eBay. I am confident another similar tent similarly priced can be had relatively quickly or order from Joe Velasko and get an even lighter one!
What a thread....as previously stated...I think everyone that has read it learned from this OP. I have had multiple bidders on my items as well and even been offered well over my stated price BUT my thoughts are: I post an item at a price and that is my target price, if it hits...it sells to that bidder by order of priority and price i.e. the first to bid(PM or post) at my stated price gets it..but that's just me (and it seems the vast majority of gear forum posters).
best of luck,