All things being equal, I think I would go for the new 1050ti. If they are the same price, then my feeling is that the seller of the used GTX 680 is asking too much.
There are many 1050ti's to choose from, with price variation as much as 15-20%. You need to decide what features you are looking for and narrow down the selection. Look for what's on sale. I would expect to get similar performance across all of them.