There are two other possible reasons why AF would not work with a Sigma.
The worst case is that the lens is, in fact, broken, and has stripped AF gears - that should be fairly obvious as it will make a grinding noise when you try to focus. If this is the case, then you should get back in touch with the seller for a refund!
Alternatively, some older sigmas do not AF with the newer cameras. The reason is that Sigma reverse engineered the lens electronics to get AF working - so missed some bits that were not used at the time (bat are used by modern cameras).
You can get replacement chips for some lenses I believe.