Air Arms Owners Club

Are there any rules etc regarding buying/selling & not being happy with the item purchased ? Thanks

as described, fit for purpose etc. In which form did you buy and were not satisfied, new second hand? look up consumer protection. if purchased via Internet from a commercial business, you can return the item giving no reason, you can even return if you simply change your mind.

I bought a secondhand stock off a guy on this site but it doesn't fit my s400 & it should with no problems. It kinda fits but its more ill fitted

There is little protection for 2nd hand stuff unless you use PayPal and eBay. County court maybe. Consumer protection is for new stuff from retailers as far as I know.
In your case you omitted or didn't know to ask if the stock with built in guard would seat the old style S400 rather than the latest one. Caveat Emptor unfortunately.