A lot depends...
First which Martini is it. Posting pictures is the best way to tell which is which.
First of all there is a whole slew of martini designs out there. There are two basics. The Martini Henry of Queen Victoria's era and the Martini Cadet that came along later and is smaller than the M-H.
This is a large Martini-Henry action
As is this
Notice the pins that go through the frame. The pins hold all the internal parts within the frame. Also notice the shape of the rear of the actions.
The next two are Cadet actions
Notice only the one pin in the front of the action and the hump back. this is a small action. Remove the pin and all the internal parts come out with the trigger group.
The large actions are perfectly fine chambered in .225. The Small cadet action has a problem with the .225. The action is plenty strong, just the barrel tenon is small. Because of the small barrel tenon the chambers tend to bulge. This makes for poor extraction and short case life. this is not a probelm as long as you keep your loads down.
If I were to consider a .225 in a cadet action I would want to test fire it. If extraction is poor then I would either pass or use that as a factor to knock the price down, way down. All you can do if the chamber is bulged is rechamber or set the barrel back.
What it's worth is difficult to determine without seeing the gun. The Action by it's self for building into a neat little sporter is worth $175 to $200. you can go up from there.
