My wife and I have been taking fresh-roasted coffee to the farmers markets 2-3 times per week since June of this year. We've stayed committed to bicycle delivery throughout, which has meant hauling a LOT of stuff on bicycle and trailer. Recently, we started serving brewed coffee, which meant a lot more weight (five gallons of water, coffee brewer and accessories, and a hand-washing setup).
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.
The bicycle trailer is built from 3/4 inch E.M.T. conduit, 5/16 x 2.5 inch bolts (no welding), and some E.M.T. couplers. For the wheel drop-outs I bought a length of thick, flat steel, which I cut into rectangles. I made horizontal drop-outs because I figured that would give me a larger allowable margin of error (i.e. the wheels can be shifted to different spots of the drop-out in order to align them properly). You can make the drop-out slots by drilling a hole and then sawing in to the edges of the hole. After making a couple of problematic hitches, I finally bought one from "Bikes at Work." It's a great hitch, although I still had to fashion and attachment from the tow-bar to the hitch, which I did using an E.M.T. coupler. Finally, I strapped a bunch of old inner tubes on it to make a comfy seat for hauling adult friends (it even has a head rest for laying down). :)
There are lots of "instructables" on the internet for building trailers, so I'm not getting very detailed here. The whole project is really cheap, provided that you have access to cheap wheels and tools (requires a pipe bender, drill/drill bits, metal-worthy saw). The hitch did run me about $80, far more than the rest of the trailer.
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.