Irrespective 15" or 16" size, etc. Both types of tyres sizes are available here in Pakistan, meeting local road conditions, and cheap as well as expensive in prices, local and imported, etc. Shopkeeper is trying to fool you by playing dirty tactics.
I would sincerely suggest to avoid selling your 16" original rims! Just install brand new tyres on 16" rims from well reputed brands, like Michelin, Bridgestone, Continental, etc. With stock size, or near to stock size, and you are good to go.