That's an outrageous price for that Sigma used. I would have probably went around $100 tops on it, for one in Excellent condition with hood and caps from a vendor like keh.com with a 7 day return policy (from invoice date) for testing it, plus a 60 day warranty. ;'-)
Plus, some of the older Sigmas like that one tended to have compatibility issues with digital (because Sigma reverse engineers the lens mount protocol and sometimes gets it wrong, since newer camera models sometimes use instructions that the cameras they were using for reverse engineering had not implemented yet. You'd have to try it to find out.
You can find some user reviews of it here (and this one does get very good reviews for a budget zoom):
The very newest Sigma model with a similar focal range and brightness, including improved "DG: coatings for digital so that you have less problems from flare, etc., is only $219 brand new from reputable vendors (and far less used), and it's going to have better 1:2 magnification for macros, too (it can focus much closer than some of the older versions that were only 1:4 macro). Here's an example:
Sigma 70-300mm f/4-5.6 APO DG Macro Autofocus Len for $219 at B&H