If a recipient of social security disability earns money at a flea market, how does that affect his benefits?

It depends on the amount of the income and whether or not the income is reportable or taxable as income.  If you have sold a few items that belonged to you at a flea market, I don't think that will affect your SSD.  If you have a business in which you go to a flea market every day and set up shop, then you are working and shouldn't be receiving social security disability, which is for people that are disabled and cannot work.