A novice seller, a £2,500 watch and offering to post it by economy delivery? You will be a target for every scammer on eBay.
You can be certain that this offer is a scam. Don't be influenced by the 400 feedback, accounts are often hijacked. If you sent them a paypal request you would probably receive a fake reply, pretending to come from paypal, and telling you that it's safe to post it. The watch would be long gone before you even realised what happened.
Returning to the use of economy post, this would give you no postal insurance. Do you realise that it it was lost or damaged in the post you would have to refund the buyer if full?
Do you also realise how easy it is for buyers to make false claims under eBay's money back guarantee? EBay would take the buyer's word against yours for any supposed faults or damage. If they returned a cheap watch from a boot sale, don't count on eBay taking your word for this either.
I've been a buyer and seller here for over 20 years, and I wouldn't even think of selling a £2.5k watch here. EBay is a frighteningly risky place to sell anything of value.
