Hello mate. as stated above i doubt it will ever happen due to the fact that we would basicalliy calling those people selling the fish bad fish keepers etc. if people are selling them as they have got too big for their tank then they should be well aware that they need a bigger tank at the buyers home. unfortunately we cant control who sells what to who its really about the buyers doing research before taking a fish on and sellers knowing what they have for sale and selling it to a suitable new home.
we have all bought and sold fish over the years and the guy that turns up for your 13 inch oscar tells you he has a 6x2x2 empty for it, but has he? without delivering the fish to the new tank you are just going on the word of people. if people lie just to get a big fish then they are the cruel irresponsible people in the buying link. many people stick big fish in small tanks knowing that its too small but they do it anyway, its heart breaking tbh but there is just no way we or anyone can control it. its the same when a shop asks the new buyer what size tank they have when looking at a red tail cat and they say, ive got a 6x2x2 but will be upgrading to an 8x3x2 within the next year or so. how many people lie just toi get a certain fish? the shop goes on their word and sell them the fish.....
hopefully forums like this can educate people as to what fish are suitable for their tanks. if they already have a tank buster in a 3ft tank then all we can do is advise them to move it on. if they dont have the fish all we can do is advise them not to buy it. what they do is their own choice, some will listen some wont but those that dont listen will have the real issues later on with stunted/ill fish that they cant move on
im know your aware of everything i have said but putting people on a list of poor selling isnt a thing that could be done without fear of any comebacks....