Back in the early 1970's my uncle had a Flea Market/antique store. The way he found most of his purchases was either by driving back areas or by cleaning out basements in older parts of town.
American Pickers seem to be buying for collectors on their clients or customers list. You also have to buy what is selling. Back when my uncle had his place old wood spoke wagon wheels were high demand. A few craftsmen he knew and sold too would turn them in to glass top coffee tables. He used Horse Hanes for the legs which my uncle also watched for. My uncle also collected old pre-prohibition bottles. We would go into the old part of town and dig out the filled up cisterns. It's dangerous to do and you don't have but maybe a 6-8 hour work window before the walls become unstable. When utility water came people threw their trash {also meaning bottles} in the cistern to fill it up.
The selling aspect you question about walk in customers Really not a good idea to do that. There would be some liability issues. Hey everybody guess who just bought.... On the under hand Pawn Stars there shop is secure much more so than a buyers home plus there would be trying to get legal releases to film them etc. Obviously they do sell a considerable amount of merchandise and were doing so before their shows.
at estate sales, I love the dirty old garage and nasty basements to go through. I use to sell on ebay also. I once picked up an White Castle mug for a buck...It was quite rare and what I though I’d get 20 bucks for went for 300 dollars. I want to find a whole case of them....withiin 24 hours of listing it was at 100 dollars. I couldn’t believe it, but the final 300 dollars ended up in my bank...