The possibility of Provo Craft selling directly on eBay would be very possible but there is no way (I know of) we can verify this to be certain that it is them.
The sellers from the Utah area may have gotten the cartridges from sale promotions of provo craft stores - enabling them to offer lower prices.
We have done several searches in the past and recently too but we always end up back to the official site... I'll try running another search later, if I find anything new I'll let you know..
All the best!