I have a few of these
Check the barrel hasn't been shortened - also that it is the correct barrel to that model, the air arms and Sussex armoury barrels are interchangeable but some are not compatible with each other.........correctly operating ratchet is important as well as ensuring the spring has not been replaced with an overpowered one which can damage the entire action. clean undamaged cocking slot is another small point that adds value.
Pictures are a must really but guide prices:
poor condition - under £100
really good condition - £200
really good with functioning autoloader - over £300
"But we have our own dream and our own task. We are with Europe, but not of it. We are linked, but not comprised. We are interested and associated, but not absorbed."
Winston Churchill 1930