I'm saying this as someone who was married to an antique shop owner, still friends with her, and know a bunch of other dealers. They're all looking to turn a profit, and keep turning over their money. Some will sell the better stuff on Ebay or at shows. It's common practice to sell things to other dealers who have a more upscale customer base. There are what are known in the trade as "pickers", people who hit every yard sale they can find, buying saleable stuff for peanuts, and then reselling to retail dealers. Others specialize in "buy-outs" or "clean-outs", where an elderly person dies or goes into a nusing home, the kids usually living in another state or just to lazy to sell the stuff. The dealer picks out the good stuff for herself, holds a yard sale for the rest, and what doesn't get sold is donated to a thrift shop. Everything from lawnmowers and old clothes to expensive porcelain figurines has to be out of the house! And yes, cameras too.
At each stage everybody wants to make money. For the most part nobody knows squat about photography equipment. They just hope to double or maybe triple their money, and if they can do it the same day all the better. One elderly lady who had given up her shop used to sell on Ebay. When she got camera stuff I'd help her with photographing everything worth listing on Ebay, write the ad copy for the camera stuff and give her an idea what the reserve should be. In exchange I'd get 10% of the money and on occasion some free little stuff such as 8 Leica cassettes. Another dealer asked me if I'd like to buy a "sack of old cameras for $500". I found out later that he was the only bidder at an estate auction and the sack went for $25. I got a few Leicas, including an M2, an M3, a CL, an assortment of Leitz lenses, plus a Minolta Autocord, a 150mm Hassleblad Sonnar, and a bunch of "little stuff". An hour later I'd sold the 'blad lens to my repair guy for a CLA on the M3 plus $600 cash. Bill was thrilled that he'd turned $25 into $500 an hour after he'd left the auction. He later told me that he suspected it was worth big bucks when I didn't make a counter offer of maybe $300.
Don't be afraid to make a lowball offer. A lot of "thrift shops" solicit donations but are not connected to any kind of charity. They're money making businesses. That pristine Pentax Spotmatic might not have cost them a penny. It might have a $50 price tag on it but they might accept an offer of $10.
Get to know what a wide range of photo equipment sells for on Ebay. You might be looking for Leica stuff but a clean Rolleiflex for $20 can be sold at a tidy profit. Always sell the "little stuff" seperately. Sell that 50mm Summicron by itself. It'll bring almost as much as it will with hood and front and rear caps and UV filter on it, but the "little stuff" might bring as much as the lens itself will.
Don't forget to ALWAYS offer your finds here on the forum first...LOL