@Blcjr, The quick answer to your question is they can and are being used in sufficient quantities to make it worth the risk. As it seems that there is no enforcement to stop it.
Counterfeit are being sold on eBay
to the the tune of 10's if not 100's of millions of dollars a year, I recently saw just 2 sellers alone that had sold about a half a million dollars (face value) in counterfeit stamps at about a 20% discount. They are having no trouble selling them.
I suspect many of the buyers are just unsuspecting individuals using them on there mail.
Most if not all of these counterfeits cannot be easily identified by the average buyer or even postal employees.
Here is a link to another seller, not one of the above, that has had $189,000 in sales with just 6 listings all various forever stamps, selling for 75-77% of face. https://www.ebay.com/sch/melvzo/m.h...047675.l2562
I will add a disclaimer to the above sellers listings, about it being counterfeits as I did not buy from them, but I will let you be the judge.
And in Ebays infinate wisdom they suddenly banned/enforced sellers from listing counterfeit stamps, I found this out when they suddenly took down one I had listed.
I called them up and talked to a CSR, explaining that they are going after the wrong sellers. I actually showed her one of the listings that should be the target, and pointed out that particular seller had a few other listings of counterfeits, and between them they totaled almost $200,000 in sales.
The CSR rep was actually very nice and understood that these would have to be counterfeits (nobody has that many extra forever stamps just laying around). I also told her there are many other listings like this, and they don't have the word counterfeit in the listing, but are being sold for actual use as postage, which is a federal crime. She said she would show the listing to her manager, but when I checked a while back it had not been ended.
So unfortunately , eBay
seems to choose to ignore this problem, as it is quite profitable for them.