Why did you order 20 devys initially? Did you buy 20 because 20 expressed/confirmed interest to buy it? Or did you buy 20 because you thought you could sell up to 20?
Everyone has bad experiences. I also had a bad experience with a seller. He said he would sell me this product but changed his mind and decided not to sell. I actually missed ordering online because I committed to buying to him. When we were supposed to meet, he sent a txt changing his mind and for me not to come. I was also mad as a customer but I can't force him to sell to what we agreed.
I think you have the wrong business model. You are trying to protect the business instead of providing a product that the customer wants.
Selling and buying for people is a different thing. Business is really difficult. But if businesses piss customers off, then it'll lead more to it's downfall once a better business opens up. If you have a monopoly, then you can survive even if you piss people off.