is barter a skill worth getting (is it working?)?
if it is what is the max you should get
180% is the recommended max from what I heard, though 100-125% isn't bad in my experience.
It does have a noticeable effect, but there's also weird variables like A) your reputation, B) some vendors generally charge more for items that are less elsewhere, and C) I could be wrong, but on rare occasions I think certain items stop having a trade-in value due to oversupply.