Looking at some hand made cards today and noticed that none that I saw stated what postage rate they would go through the postal system at.
Many cards had applied decorations that could make their thickness push them into Royal Mail's Large Letter band.
I would normally avoid such cards as I don't feel the extra postage expense is justified for cards (Ok, I'm tight).
Commercial cards usually indicate their postal rate on them.
I, and maybe other potential buyers, would be happier buying if the postage band was stated on listings.
Just a thought; shoot it down if you want.