Selling a product at different prices to different customers

What is the ruling on charging fees in return for delaying payment until the time of delivery when buying online?

Ruling on buying apartments and shops before they are built, and on getting a discount if you pay in full up front

Is buying expensive items regarded as extravagance or excess?

Ruling on extra credit offered by phone companies

Ruling on selling phone credit worth 10 pounds for 12 pounds

He bought something on the Internet but he did not accept it because it was defective. Is he obliged to pay the shipping costs?

If the seller is a group of people, and they keep the down payment after the purchaser cancels the purchase, how should they divide it among themselves?

Ruling on putting a car on the market and offering it for sale in the auction, when he does not really want to sell it there; rather he only wants to find out its price

Ruling on paying for a product by cheque at a higher price