Whisky, like all imported spirits in Tunisia, is expensive if bought locally but traders have long got wise to the fact that British visitors trying to barter whisky for leather goods have probably paid very little for that bottle of blended whisky in the duty free shop at their departure airport and don't like being taken for a ride any more than British visitors like being ripped off when they discover they've paid an inflated price for something in the medina compared to what a local would pay. By all means take the chance if you want to but don't complain at the grief you'll get if the trader realises you've offered them a bottle of whisky worth a 10th of the worth to you of that leather coat you've been trying on.
Frankly I think that the 'Bottle of whisky in exchange for a leather coat' story has now achieved the status of being an urban myth. It's always a 'friend of a friend' who's done this these days rather than the person themselves who tells you the story. It might have been true in years past when few British visitors went to Tunisia but I suspect that these days many traders get pretty fed up with the umpteenth tourist who tries this on with them and then gets annoyed when the trader refuses to be exploited in this way.
If you want a decent leather coat at a bargain price compared to what you'd pay here, check Briar's earlier posts on the subject and pay for it fair and square at the manufacturers in Hammam Sousse secure in the knowledge that you've got a quality product for far less than you'd have to pay here and that neither you nor the trader have been ripped off.
SM