stampsplanet has an
ebay 14-day return policy. High volume seller with good feedback, but the mint bundles could still have counterfeits. I also don't know how may fakes USPS catches so buyers could be shipping w/o any indication they are using CF. It is pretty clear from feedback the buyers aren't opening their packages and immediately recognizing them as fake....an ....they probably just assume good and either use them (seems most popular use case) or keep some.
It seems anyone buying would need to do some tests (have comfort they were USPS sourced) and assume the worst on any forever stamp purchase.