Those who buy apple products from Amazon or any other e-commerce site are having problem with the performance of product so can you solve this problem?

If the device was purchased from an Apple Authorized reseller Apple will provide service and warranty. If the seller was NOT an authorized reseller you will have to deal with the reseller and whatever support they offer.

If you buy something from an online site other than an Apple site, you have to deal with that site. Apple is not responsible for products purchased from those sites. You can still take a phone to an Apple store for service if there is a physical issue with the phone.

Apple does not sell products through other e-commerce sites. The other e-commerce sites buy phones from Apple, who cannot refuse to sell phones to anyone willing to pay the price, they then sell the products. May e-commerce sites are authorized resellers, but not all. Amazon itself is actually an authorized reseller, however there are 3rd party resellers hosted on Amazon who are not authorized resellers. In the US large businesses like Walmart, Best Buy, Microwarehouse, Frye’s, OWC, etc are all authorized resellers. And all US carriers are authorized resellers. In other countries there are also authorized resellers, such as Tesco, KRCS, Carphone Warehouse and others in the UK.

Apple does not sell products through Amazon directly. Some company or individual buys the item first and then resells it.