I'd say you can leave the "Selbst-" bit out of the headign; it only means they are not "marketing" for other organisations, and this will be pretty clear in your text, I'm thinking.

Looks as if it's rather more than just a farm shop, so "operation" for "Betreib" should be well justified here.

farmers' direct selling

More context would help to fine-tune the expression.

"Selbstvermarktung" means that the farmers sell directly to the consumers not to distributors, food producers, retailers.