I'd be asking for a full refund, no comprimise, the 28-75's come up fairly often on ebay,got mine for $340;00 with CPL and ND8 filters. Any scratch or defect at all pretty much renders them worthless on the secondhand market which may not effect you at all.

Probably dont need this advice but.....I never..never buy anything from anyone unles I'm 100% sure they are reliable, you stated he is notorious for lack of respose...avoid dealing with these jerks.

PS : the Tamron 28-75 is a bit soft wide open and at full reach, but to get better IQ you would need to be buying pro grade lenses at well over $1000.00....still a very fine bit of glass. Mine has a 5mm scratch on the rear element caused by me , but never shows at all in images, but I hate it being there