I've always made it a point to make sure any instrument I've sold was cleaned, polished, lubricated, and problem-free. A buyer can't complain about dents that never occurred or dents that have been properly removed. CYA is a good thing, and sometimes it comes at the expense of not making as much of a profit as one would like. Sometimes you break even, or you may take a loss, but you always end up with more money than you would be able to use if you kept the horn.
Relisting an item on eBay can result in a different set of bidders, and you may have the good fortune to sell this trumpet for more than $300 the next time around. Of course you might have bidders that question why you're relisting, or you might hit a lack of enthusiastic bidders. A reasonable reserve should protect you. Also, a BIN listing exposes merchandise to potential buyers for a much longer period of time, so that may be the way to go instead, and you can always reduce the price incrementally until a watcher decides to buy..
Unfortunately, shipping costs can prevent a reasonable sale in the first place, and lest we forget, eBay takes their cut of the total amount of the sale, including shipping.
I hope everything works out for you.