Last year Wikispaces was acquired by TES Global based in England. One of the services that TES offers is a marketplace for teachers to sell materials to other teachers. On the Wikispaces blog I just read that TES is expanding that marketplace to include teachers in the United States. If you want to get into the TES Resources marketplace, you can apply here.
I’ve never tried selling lesson plans, printables, or other similar materials, but I know that some people have made a nice supplementary income on the Teachers Pay Teachers platform. For teachers that are inclined to sell materials, the new TES Resources platform could provide a good place to earn a little supplementary income.