Of lower age goods, yes, kinda - there's more of them out there probably than anyone realizes - but they're not "available" to be obtained because the players with them aren't actively looking to get rid of them. They're just floating there idle
But higher ages there is a distinct lack of scarcity.
And whether you view treasury as the sink or using the treasury as the sink, it's still ultimately a sink - either your guild is using them and they are being sunk somewhere, or they're not using them and they were sunk just by being donated
They can't be repurposed into anything other than treasury goods and treasury goods are generally much easier to obtain than "loose" goods (get someone in the age, level their treasury GBs).
The age limitations some guilds added were not just a reflection of market availability, but also that their best-developed-treasury-contributors were not in those ages. If you had someone with level 100+ arc, observatory, ai core, and atomium, you'd probably be fine with having them camped in some low age with it - even considering them an asset allowing you to recruit other players in that age, and run GvG campaigns in that age.