can someone get away with saying they never got the product x ?
No, but they
can get away with returning the product while claiming it is defective, and they can obviously prove they returned it. They get their money back and you get your product back, so everyone's even. Except for your bitcoins, of course, which you'll never see again. Oops.
How about if the product isn't being shipped its a virtual product that has a proof they've gotten it all documented.