The Fett looks like a UK catalogue piece - KAYS catalogue judging by the reference on the label. Don't worry about the gun missing, it's one of the pieces you can pick up separately if it's missing - normally about £20-25.
The box and figure look to be in really good shape, plus it has the yellow insert. Do you have the 2 instruction sheets that go with it? Check inside between the box and insert, they might still be in there.
An example like this has not been sold for a while - 12 months ago they were fetching £300 all day long - I would hazard a guess that has gone up, so maybe £350.
If you've got any doubts then keep it - you can only sell it once and then it's gone for good. Most of us here would give our right arms for our childhood collection. You've got the making of an awesome collection there, I'd be keeping it but if you were to sell then start thinking about a nice holiday...