I've done it, but only from trusted sources. Can you return it? Are they a dealer or an individual?
Another thing to consider is cost of retipping. If there is something wrong with it, or it's worn, can you have it retipped and still come out way ahead of the cost of buying a new cart? One of my used purchases was a Van den Hul Crimson XGW Stradivarius, which at the time were fairly inexpensive to have retipped by the manufacturer. I paid less than half the price of a new cartridge and have not had any issues with it to date.