I have been an "ebayer" since 2002 and love the unique items found on ebay. My husband is a USATF (USA Track & Field) athlete who receives shipments of USATF uniforms prior to every major international race. Because he doesn't wear much of the gear that is shipped to compete in his event, we make it available on ebay to other track enthusiasts.
Also, since many of his teammates train full-time, which makes it difficult to hold down a full-time job, I sometimes offer their gear at auction to help them get by so they can continue the athlete's lifestyle.
Most questions we receive regard sizes. For reference, my husband's dimensions are 6'1" and 170 pounds. I know it's hard to purchase clothing without trying it on. The best advice I can give you is to go to a sports retail store and try on a similar item made by Nike (or the manufacturer of the item on which you would like to bid). The size/fit you like best is your size for our auctions.