I think $50 for a used 6 inch dado sounds a little high, given that about $60 or $70 can get you a brand new 6 inch set though. For what its worth though, for a dado stack I'd wanna go up the the $100ish price range and get an 8 inch set. Sure, you probably dont need the extra cut capacity, but that one time you do you'll be grateful to have it. In addition, the teeth on an 8 inch set will be moving faster than the 6 inch, which results in a faster, cleaner cut in my experience.
I like the dewalt 7670 stack. 8 inch, runs about $100 depending where you buy, comes with a swanky carrying case and cuts like butter. Personally, I couldn't bring myself to trust a used saw blade no matter the brand. To many unknowns there that could result in a poor cut, or even worse, a tooth dislodging