To your exact question I would say you should not be allowed to increase the price since you didn't allow your buyer to do so...
However, I am highly against anyone (not just artists) thinking they have any right to dictate what a buyer does with anything after it has been paid for. At least you allow your buyers to recoup costs for purchased art, though.
As an example: if I buy a painting from an artist for $50 and that artist later becomes famous and I can sell that painting for thousands then that artist has no right to say "Nope, $50, that's what I got paid for it"
I understand why some people do it, but I would never buy anything from anyone with such a policy in place.
I've thought about that rule, but I somewhat encourage it art-wise (like if someone bought something from me for $10 and got $10 worth of art made of it, I'd prefer if they sold it for $20) just to encourage them not to hoard my characters for a profit since I work hard on them to see them go to good homes
But I think it mainly applies to common artworks like what you see on most of DA