My credit cards regularly offer 5% cashback for grocery stores, drug stores and other physical locations that sell gift cards. They also offer 5 percent back for pay pal purchases which can be combined with places like Raise.com to get a lot more than 5 percent saved in total, and gift cards almost always go for under full price.

If you're doing an online purchase you can combine with a coupon collector site, credit cards, and gift cards to get steep discounts. When I shopped at H&M their gift cards regularly had 15-35% discounts from gift card sites and they will accept any piece of fabric in store for a discount coupon, making the clothes close to fifty percent off in total.

When shopping online it adds possibly two minutes extra to check out once you get used to the flow. It's not recommended for gifts as it mucks up the return flow but gift cards are usually only for large retail outlets so it's always possible to hold on to it and purchase something later.

This is all legitimate use and not a flow for money laundering.

If they're using fraudulent payment sources (stolen credit cards) it may be harder for them to use them on Amazon versus somewhere else.

The buyer may not have supported payment methods, especially overseas.

E.g. most German cards don't work at amazon.com (unless things have changed recently), but work at PayPal.

I wouldn't, and haven't, bought anything from Amazon since Amazon facilitates and perpetuates fraud.