The problem is you put something with low demand into the daily dirt to advertise that your farm needs help, then someone buys the advertised item.
How about putting - say - an item like a flag with an explanation mark into the paper at cost 1. Fix the item so no one can buy it. The item vanishes from the shop when no help requirement remains ... Maybe it goes back into the barn for re-use.
This would be in addition to the flags on buyable stuff

