As long as it's not gray market, I'd say it's a pretty good deal. I've bought from buydig before (though not through eBay) and had a great experience. I purchased the Sigma 50mm f/1.4 for a good price. The first copy had AF issues (like many copies of that lens), so I sent it back. Their return process was a breeze, and I received my new copy a week after I returned my original.
I'd definitely recommend them if you get a great price.