Napoli don't need t sell Cavani, his contract isn't expiring next summer and they"ve just sold Lavezzi for an stupidly overinflated price (yes, I'm not a fan). And Llorente as a Cavani replacement would be a huge step down for them.
Also, why would Llorente exchange Athletic for Napoli? It'd be a step down for his as well. The point is, Napoli have arguably the best striker in the world and when they sell him, they won't be able to replace him with a striker of similar quality. That's why they'll try to delay his sale as much as possible IMO.