I've tried to sell the berries, still on stem, but not the flowers. What are the uses for the flowers?

I've never sold the elderflowers, but make a beautiful liquer every year (fantastic in champagne!)

Do they hold up after cutting them off the plant? Do they look good in a vase of water while you're at the market? If not or if they wilt, then I would focus on folks that home brew their own concoctions with them (they are actually very popular but I think most folks just harvest them from the wild). If you are selling at a farmer's market and they display well you might sell a few to folks wanting to experiment with them (they make good floral flavored fritters also) but I wouldn't think they would be big sellers. You might also find a wine/beer making shop and either sell them to them or at least post a notice that you have them available for home brewers. I would price them like any other flavoring herb except that it takes a lot of them to make a strong tincture and factory made tinctures don't really cost that much. So charge whatever you can get but don't expect to get rich from this crop.

Thanks for the suggestions. I'll check out home brewers.