Earlier quoted context omitted.
Note that eBay provides this sort of distinction, in the form of feedback history (someone who's got feedback from 100s or 1000s of transactions is probably making money at it), and people generally prefer to deal with the dealers and middle-men (in part because dealers have more to lose if they rip someone off and get bad feedback). [edited to clarify wording slighty]
Feedback histories show that eBay dealers are active users with the greatest number of transactions, and that those with the longest histories are successful because they continue to conduct themselves well. But it doesn't say much of anything about whether individual buyers or sellers anywhere (but especially Craigslist) would prefer to transact with dealers rather than other pedestrian users when given those choice…
3 years ago I was broke and bought stuff on craig's list, (specifically camera equipment) and sold it on eBay and held some and sold it on craig's list later. I negotiated hard on the phone and only bought stuff far below market value.
I only sold stuff for far above market value and I ended up with an extra $300 a month or so which was a huge difference in my lifestyle.
I noticed something with the sellers I dealt with. Most were wealthy, or made very good incomes and just wanted to get rid of the item. They didn't want to deal with a bunch of buyers, they wanted someone who would give them cash fast in a straight forward transaction. That is the "service" I did for them.
Buyers wanted the item, but wanted someone who would tell them it worked, and back it up, provide good photos and description etc.
--- Fast forward 3 years and I make enough money that when I buy and sell stuff on craig's list I am primarily interested in a fast, easy transaction where I'm saving money over the new price, but not concerned about getting an extra $20 out of it.
I basically changed places and I am happy that middlemen are able to provide me with fast, easy transactions.
--- One tip too. To get the absolute best deals put up WANTED ads on specific items. I bought stuff for ridiculously low prices that way. 60% off the mean. And you don't have to be the first to respond to the ad. A lot of people search for the item they are about to sell before they post it and when they see your WANTED ad, you provide the service they want and they are happy to get less than the item is worth.