All About Pocket Knives

A friend of mine picked up a Gerber Applegate-Fairbairn folder(assisted opening)with a black blade. He wants to sell it to me and told me to make him an offer.The blade had been sharpened several times but seems to be in decent shape. My concern is with the locking mechanism.When pushed forward to allow the blade to open it doesn't stay in position and makes it very awkward to open the blade.Likewise when the blade is in the locked position closed it slides forward slightly.When the blade is open the button has a bit of spring tension on it to keep the blade locked.
Doesn't seem like it really functions very well.Checked e-Bay and the prices are all over the map for them.Most of the e-Bay knives have bright blades whereas this one is black. Is this an earlier model?
Any idea what a fair price for the knife would be? I want to be fair as he is a friend and I would like to keep it that way. Here are a couple of pictures. Any advice would be gladly welcomed.