​I wasn't a disgruntled artist, I was a successful one, and I ran an art supply store within my multi-award-winning art gallery. That store sold more high-quality art supplies during the time I managed it, without any theft from staff—as having ethics, I attracted loyal ethical followers. 
The reason my art store sold more of the quality supplies than any other in my state in Australia at that time was that I gave free advice with a purchase. 
That worked for decades—then, with internet growth, it changed. Artists would come to me from outside my area, pretended they wished to purchase materials, asked questions, sort an art demonstration of the products, literally conned me into giving them a private art lesson. Then, having worked out what they wanted, they walked out the door and no doubt ordered the goods from a discount online store.
I closed the supplies store door to all aside from the artists I taught. I have no time for people who mess you about.  
Being a poor artist doesn't excuse a lack of integrity. I'd never con a storekeeper like that, nor steal from them. 
Honesty doesn't cost anything—even poor artists can effort to treat store owners with respect.