They always do so in the South as some already pointed out here.
“I bought a fairly decent sized water melon last month for Rs. 100. This was in Punjab, where water melons are priced by visually inspecting their size. In Sindh, they are sold by weight.”
http://www.paklinks.com/gs/all-views/432807-what-is-the-current-price-for-a-melon-in-pakistan.html