Should I buy a used GTX 980 STRIX for $500 CAD?
YES, if its operational and in a good condition. ;)
Yes, because it will run like a GTX 1060 that aren't available for sell right now anyway. ;)
Personally i wouldn't, i'm not sure about Canada's used market for 980's but before prices went way up 980's in the us where about half ($200 usd) what that person is asking, 1060 6gb's will preform better, use less power, less heat, have warranty along with being new and the same price if not cheaper.