The core issue that I can see (aside from the prices, I mean holy shit I would not pay ~£120 for an amateur piece of canvas art) is that they are very amateur. Lines that should be straight are not, parts that should be symmetrical are not, and I question some of the colour choices (pink background for Vader? Why?) I'd like to hear the artist's opinion on these.
Continuing on the theme of the backgrounds, I'm a little confused by the Goomba. Now I despise Mario, but it appears to me that the artist either didn't think much about the background, or wasn't bothered, as it seems to be standing somewhere below the grass and a layer of topsoil. Also note the uneven lining of the tube behind it.
I also take issue with the description. It is claimed that it was 'inspired by the Goombas from Super Mario'. No it wasn't, it's an actual fucking Goomba.
And as a final point, where would Nintendo et al stand on the whole thing of you using their copy-protected things to make money?
I'm not going to decry your artistic skills, I just don't think they're good enough to be paid the amount you're asking.