Why can't I use my Mastercard Gift Card on Ebay or Amazon?

I checked on the Mastercard website, and it says that the gift card can be used for online transactions such as shopping and paying bills. I tried to buy stuff off of Ebay and Amazon, but I keep getting error messages saying that it can't validate my credit or debit card. ... It might also be rejected due to address verification service (AVS) â€" since the gift card does not have an address attached to it, the electronic payment gateway might be denying the transaction. ...