Welcome to the bbs Mike.
There have been a few FWB mod.80 sales here on the bbs. £180-275 seems to be the price range, yours being near the top of that range owing to it's condition and assuming it has all the tools, manuals etc.
A mint one sold on Gunstar recently for £250 complete with box/tools etc it had just been factory serviced.
If your's hasn't had the original seals/buffer replaced then you should factor that into the price. They degrade over time and should be replaced to prevent any damage occuring during use.
Here are a couple of links to old sales posts: (Sales access requires 4+ posts, so these links may not open ~ get posting!)
1. Excellent+ condition. £270 asking price.
2. FWB 80 £220
3. LH mod80 £185 asking price.
As for your second question re. a pistol for vermin control. I think there's a general view that air pistols aren't suitable for that job.
If you mean dispatch of caged/trapped vermin there are better ways than using a pistol. A cheap .22 air rifle is effective and you can poke the barrel through the cage material.
Hope that's of some interest, and welcome to the bbs.