Hey Tom, One thing that I do when selling designer goods, is I put a close up photo of my receipt for the item in the description. If the shirts are legit, the sales receipt will show that and ebay will normally leave it alone.
One bad thing about listings on ebay is that any one looking at the listing can hit the 'report' button at the bottom of all auctions and ebay will pull that listing. They kinda have a 'shoot first-ask questions later' method.
If they have a 'vero' complaint about your listing, they will normally pull it no questions asked. If someone hits the 'report button', the ebay staff will look at the listing and make a decision, based on what they see.
A 'dirty trick' that some greedy sellers use to help 'limit' the competition is to just hit the report button to get the listing pulled. New and low feedback sellers are normally targeted.
Just be very careful because ebay will suspend you for 7 day normally the first time, if they have to pull your listings, then the next time might be 30 days,, but the third time could be indefinate.
You might also want to contact ebay and provide them with proof of authentic merchdise, suchs as receipts, that prove you have legit shirts and also of your intent to list them. It's a bit of a hassel, but once you do that, they normally won't mess with you..
hope this helps..gulfy13