A key component to accomplishing this has been a trailer that I built specifically for this purpose, designed such that my widest item (a fold-up table) would fit between the wheels. Below I discuss how I built the trailer, in case you want to give it a try. First, here's a video of me and my brother-in-law setting up our market booth with supplies carried in by bicycle power.
I will note one detail of how I built this, which is different than any instructions I found on the web. Note how the main part of the frame consists of two oblong rectangles, which are bolted perpendicular to each other and form the structure of the frame AND the wheel wells. This means that you have to make drop-outs of different lengths to adjust for the fact that the two sides of the wheel well are at different heights (because one is bolted on top of the other). This is an easy adjustment and I think that it makes for a stronger trailer to have the wheel wells so well-integrated into the frame.
The trailer has performed great all summer (notwithstanding some failed experiments with home-made hitches). My wife can easily haul me (150 pounds) on it and I was easily able to haul my 200 pound friend. The weight that I've hauled for market is unknown but seems to be much heavier than the 200 pound adult male, so I'm ball-parking it at around 300 pounds.