"You may have seen the "Bartzella" seeds for sale on ebay? I wonder how they get Bartzella to produce seeds?"

Yellow "Intersectional" Paeonia Peony Seeds on Ebay
Actually I haven't seen those. I just looked them up. I believe those seeds are a hoax. My Peony 'Bartzella' has never set any seed, and I'm guessing yours hasn't either. The seller is likely playing into the fact that Itoh peonies are generally expensive (although their prices have come down quite a bit already). So they are trying to sell these "seeds" for less than the cost of an intersectional root, but highway robbery, nonetheless. I definitely would not purchase those seeds, as they are likely some seeds from an undesirable, common peony and will most likely not be yellow at all. Buyer Beware for sure!