Spyderco and Benchmade as well as Cold Steel are good sellers and have ready customers looking for good deals. Get to know what to have on your table and what not to bother with. Hot "super " steels, knife releases, sprints with different steels or better scales are sexier and go faster than plain vanilla stuff they can buy anywhere. Note that it doesn't always mean too expensive. It's what's hot, not what's expensive. Not many will buy super high end stuff unless there is a reason to. You have to know what is in demand and what isn't.
I used to do what you do but I just sold at gunshows and not flea markets but I had full time work so it was more a hobbyist thing.
There are lots of others. Find a niche for yourself. I used to put up a sign listing brand names and another sign that listed different steels. If you collect knives yourself then it is easy to work with other dealers helping each other out. I bought knives from other dealers on occasion and got dealer rates but I returned the courtesy when I had the opportunity.
Now with very convincing fakes so available expect some to request some sort of provenance showing the authenticity of your name brand knives. Don't be insulted, just work to make the potential buyers satisfied their hard earned money is buying real product. Once you develop a good reputation things begin to get easier.
Don't keep fakes around. If you need counterfeit products to stay in business you are doing things wrong. Whatever money you gain will not make up for the business you lose from folks like me who just won't consider doing business with you for any reason.
After you begin to network with other dealers you will then be able to find out how to share on the deals, sales and finds of NOS that enter the market occasionally. I've seen really cool things reappear years after they were no longer available when someone sitting on a pallet full decides to get back some of the spent money rather than keep them around another year in the wharehouse.
It can be fun but the food can be terrible.

Good fortune.
Joe