So I bought a Glass Paint Brush today at a great price and can either sell it or save it... Will it gain value because it is retired or should I sell it now?

The glass paint brush has become a useless item, so I can't see it gaining much value. Personally I would sell it, but only do what you feel comfortable with.

I agree with Slavecrown. I think you should sell it. I don't think it will gain a significant amount of value anytime soon. It's been retired for a few years, and can still found in the shops. Best of luck with your decision. =)

 

Here's an NT article for extra reference:

http://www.neopets.com/ntimes/index.phtml?section=34361&week=172