there are some china compound rubber that they make seals out of that dont like our gas. even china fuel hose.I put new china fuel hose on my 6 chain saws( huricane supplies) and...withen a hour the hoses were funking up the carbs or spliting,popping off. I suppose they are ok somewhere else in the world...so..as to your ebay tank...mine were ebay and directly from china hoses... the china aluminum an fittings leave a lot to be disired, but so do the amreican ones too....boy do i have a story about a pile of very expensive american an fittings that were junk..real engine/ fuel system killers...earls I think,hard to remember after buying oh somany of them, aeroquip was always the best. to make it short dont ever buy the aluminum " stainless look" fittings.I think there off the market now.they were aimed at exzotic fuels..and fools...apparently. the "stainless" coating came off in oh so needle size extra hard sharp slivers, lot of them...on the inside of the fitting..to go into your engine..and fingers too.
as for the AC aluminum most car garages/repair shops that do ac have them in a pile waiting for the scrap guy. lots of usable stuff in the scrap piles these days.