What is the ruling on selling computer maintenance warranty cards for a period of two years, and does its ruling differ from that of mobile phone maintenance warranty cards?

It is permissible to sell a product with a condition of maintenance warranty for a specific period, and it is permissible to increase the price of the product in exchange for extending the warranty period, because in this case, the warranty is subsidiary to the sales contract.

However, selling independent warranty cards is not permissible, due to the element of gharar (uncertainty/risk) and gambling involved.

The prohibited gharar is that which affects the primary subject of the contract, not its subsidiary.