When you go shopping in China most shops will not give you a receipt by default, the reason being that they then have to pay tax!
When you ask for a receipt, often referred to as "Fa Piao" in Chinese, the shop will increase the price by at least 5%. For waranty reasons, you do need the receipt, as many manufacturers use the receipt when repairing electronics under warranty. This is especially important if you buy a Dell Computer or an external hard drive for backup. Dell computers could be in the shop for several months before you buy them, but the warranty began when it was delivered to the shop. Please be aware that you have to go online to initialize an "Owner Transfer", can be found on Dell's homepage. External hard drives follow the same structure, there is a 1, 3 or 5 year warranty from the date of sales. Hard drives normal warranty, without a receipt, is from production date.
Please have a look at the pictures below depicting a real "Fa Piao". The receipt can be spotted by checking the oval stamp on it. The stamp should have a number in it, which is the company license number.