Good piece J.
Just to add to what J has written, as he points out, markets will have a core group that will visit virtually every week, and they will usually make up the most of numbers.
Whether this is a good or bad thing will depend on your style of booth, for example....if you sell doughnuts, the more regulars the better because if your marketing is good you stand a good chance at getting those more regular visitors sooner or later.
If you have a booth selling products that doesn't have the variety for you to change up with new products every couple of weeks, then you will usually find your sales will peak in the first quarter as the regulars feat on a new thing, then start to die as they have all been there and done that!
If you are considering selling at a market, then consider the product you are going to sell, because you need a way of generating sales every week and you are going to have to turn those regulars into your core customer base if you are going to have prolonged success over any given amount of time.
Do some research, visit the market a few times, talk to some stall holders about their products and sales. Take notice of face in the crowd, where they there last week?
Identifying your market is the basis of any good business, and part of doing that is matching your products to customers. Not much good trying to develop a marketplace where you have products that customers aren't going to want after a few weeks : (