The thing with Sigma and Tamron lenses in my experience, is that they CAN be very good in terms of sharpness. While their AF motor speed / sound, and construction may or may not be as good quality as the original Nikon / Canon lenses, I've had a few great copies that were almost on par with their name-brand counterparts and have tried some that were soft all around no matter what. You just have to test out many copies to find the right one since the range of quality for Sigmas and Tamrons seem to be quite vast, whereas Canon and Nikon have much better quality control.
I just bought a Tamron 17-50 (Canon mount) for my girlfriend after trying out 3 copies. A great alternative to the Canon 17-55 version that costs 2.5-3x as much haha.
Oh, I'm sure you already know but you shouldn't buy from "unknown" sites, there are a lot of scam stores that show up. A pretty good way to check is to visit resellerratings.com and search that store to see if it's legit.
2007 E92 335i, 6MT, Sparking Graphite, ZPP, ZSP, Nav, Logic7