I think you have a case.
Since you specifically asked if the goods were likely to be on offer again and you were told that they wouldn't be, this statement may amount to what is called 'negligent misrepresentation', since the seller appears to have 'failed to take reasonable care in making the representation' [and] 'had special skill or knowledge, and the other party was reasonably entitled to rely on that knowledge, which turned out to be false'. (Woolman & Lake, 'Contract', third ed, page 81) discussing Esso Petroleum Co. v. Mardon [1976] Q.B. 801. In short, the guy should have at least checked for you and if he had, he'd likely have found it was due on sale again imminently and was obliged to tell you.
Your remedy is a refund as the contract is either void or voidable - if the seller disputes the contract being void. This would then be a matter for a Court.
However before taking Court action, I would advise you get trading standards involved. As far as the seller is concerned they are much bigger and scarier than you are.
Secondly, because you purchased the goods using your credit card (and the goods cost more than 100 pounds), Section 75 of the consumer Credit Act 1974 (equal liability) allows you to claim your refund/damages from the creditor, supplier, or both. So if the seller gets out of Dodge, you can sue the credit card company due to the seller's misrepresentation under the Sale of Goods Act 1979.
See your local trading standards or a Citizen Advice Bureaux.
One more thing: Don't be 'too' nice! You tried that, and they threw it back at you - so get stuck in. Speak softly and carry a big stick (the stick being the Law). Let me know how it goes or if you need further help.