$700 is a crazy lowball offer for an exhaust used for 2k (if that's accurate). The going rate for a used one is about $900, I sold mine recently for $800 with roughly 10k miles on it and a few scrapes and dings.
I've never seen a used garage line sell because a new one is only $500 or something
AFAIK.
Chances are when you sell the exhaust you'll recoup almost all that you spent for it if you're buying use. These items aren't really going down in price. Know how items go down in price? People entertain low ball offers and soon that becomes the norm. (Hint: That is not a good thing)