A quick Google search will tell you quite a bit about this gun, a modest Spanish boxlock.
The proofs on barrel and action flats will also tell you or us, if photos are posted.
The market for undistinguished 12 gauge Spanish boxlocks a is very soft, so $300 is about tops for this gun.
If you like the gun, and it works as it is supposed to, you could have fun with little invested, but not expect to make money on it down the road.