The hammer could be stiff just because the wedge is push in too far, and the grips probably aren't too bad. Or it could only need some oil or a cleaning.
The determining factor would be it's overall condition and how used it looks.
If it looks well used then $225 is a fair offer. If it looks like it was only used a little then $250 - $275 isn't too far out of line.
If you offered $225 and he counters with $250, then you would need to judge how much it has been used.
Sometimes ASM parts aren't all that easy to find, and you can mention that to him.
It's hard to say what it's worth when we can't see it.
When folks buy them on Gunbroker then they also have to pay shipping which adds to the cost.
It's usually best to make a low offer unless it looks really good overall and you really want it. Then you don't want to insult him either, but your lower offer will serve to feel him out.
Then you can always tell him that you need to come back with more money, and only bring $235 with you.