If you do a simple google search on 'hide my ip', you will very quickly learn just how easy it is to not only fake your location through proxy servers but also to place your IP in fake servers. Luckily, fraud prevention software like Signifyd exists that can tell you if a customer is masking their true location by using an IP address.
My two cents beyond utilizing fraud prevention services is this, is the other order elements consistent. Does the billing/shipping match? Is it an unusually high order? IP address is only one part of an order, and determining a valid order vs. fraudulent lies in the ability to identify a consistent pattern