How/Where did you sell them? I am one of the millennials you mentioned and would love to know where I can keep an eye out for this stuff once I can afford it.
There are only a few fine art galleries, and there are even fewer cities that are fine art markets.
I sold the belle epoque art nouveau works on park ave in nyc, but I originally found them in san francisco - which is not really a fine art market but has a couple galleries
In my experience, things like Art Nouveau and Art Deco are much more common in Europe, especially Belgium, France, Germany, and the UK. The places where this stuff was born can still be visited.
In Brussels, there are antique dealers in the Marolle area that specialize in this stuff, and they're happy to help out a prospective buyer.
I’ve seen ‘galleries’ all over. How does one know if it’s really a fine art gallery or not? I have a pretty good eye as I’m a novice painter with a few years of old-style academic training under my belt (casts, nude models, etc). All I know of that I’d call mostly respectable are Maxwell-Alexander and Arcadia Contemporary. But they deal only in living painters.
Honestly it’s about what other galleries think of them
The key term here is provenance, how strong and reliable is proving the history of ownership if that gallery is involved, can they produce the prior provenance on demand
I don’t know the best way to find that out from the outside. If you try to sell a piece you own you’ll find it very quickly, after you mention where you got it from
