Go for it leave1

Sigma is a good lens and you can't complain at that price. But try this test first, just in case it's been traded in as a duff copy, or been dropped or whatever.
Stick it on your camera at 10mm and f/2.8, say ISO 400 to get a guaranteed shake-free high shutter speed. A bit of noise won't hurt. Point it over the road at a reasonably distant subject to minimise focus inaccuracies, a shop sign/number plate is good, and focus carefully. Set exposure, lock everything on manual. Shoot four pics in quick succession so the light doesn't change, with the target subject in all four corners of the frame.
Take your card and pop it into one of those processing booth things, select the target subject in each frame, whack it up to max magnification and print them all out. A 6x4in section is perfect.
What you are looking for is four corner images that are EQUAL in sharpness. In truth, they probably won't look that great but this is a severe test and it's equality you're after, not ultimate sharpness. You will probably find that the four images are not quite the same, they rarely are even on the best lenses so don't panic unduly, but if one or more corners is obviously significantly worse than the others, the lens has been put together off-centre (the most common manufacturing fault) or has been knocked off-centre.
I say again that this is a severe test, pushing the lens very hard at its weakest settings. So why not also take another shot to see what it really can do. Set a mid-range zoom focal length, f/8, low ISO. Print that one 15x10in and prepare for a big smile
Cheers,
Richard.
Edited to add, if a lens is really duff, you don't even have to print the images out - just blow them up to max mag on the LCD screen on the camera and you'll be able to see if one corner shot is way different