I had a co-worker who did exactly that. She was something of a dabbler, she had a booth at one of the local antique malls and did pretty well and one day she bought a piece of Cambridge Glass for $300 and turned around and sold it for $6000 dollars and decided to pack the whole work thing in.
She was very knowledgeable about glass. I think that it’s probably a good idea to find one or two or a few areas of speciality and narrow it down to those specific areas. I have a friend whose mother was an antique dealer and her main area of expertise was antique toys, though she managed to do pretty well with some other things too. There is simply too much stuff out there to make it possible for you to know everything about everything and you have to be able to “separate the wheat from the chaff” if you want to make any money at it. There are too many fakes, frauds, reproductions out there and too many variables not to specialize, I think. I’m no expert but, for example the same piece vintage glass or pottery, the same shape, pattern and style, and produced at the very same time could be worth $10 dollars if the color is red and $1000 if it’s in peach just because millions of the red ones were manufactured while only a few thousand of the peach ones were manufactured. Concentrating on a few area seems like it would at least be a place to start. I think it can take a very long time and a lot of experience on top of simply just having a knack for it, as @Nullo put it “having a good eye for antiques” I think that really does play a big role in being a successful antique dealer; I don’t think you can underestimate the value of that, ” the good eye” at all.
@Nullo LOL. I like the “food chain” analogy and what you describe seems to be an economic model unique to the business of antiques and vintage collectibles. Generally someone manufactures something, we buy it, we use it until it breaks down or is no longer useful, then we throw it away. The same vintage Fiestaware pitcher could be bought and sold and bought and sold, over and over, cycling and recycling it’s way through garage sales, thrift stores, flea markets, antique shops, estate sales, etc. in perpetuity, or at least until someone drops or chips it, probably making someone at least a little money each time it changes hands.