Cancellation fees are kind of a problem, for reasons stated above. I do believe artists need some form of protection, though. Art is a rough business. As an artist, your time often gets exploited by so-called customers, taking more requests than making actual purchases, resulting in murderous concurrence and a lot of wasted time, which is rarely compensated by good enough pay. I think it would be a good idea to provoke forward payments after a wip is shown and the consumer agrees on buying art from the artist. To elaborate on this, here's how the process of buying art would look like:
Consumer opens a request thread with detailed info about what they want -> Artists offer their services, showing a portfolio -> Buyer agrees with artist on the price beforehand (forward payment as well as payment afterwards) -> Artist delivers wip -> Buyer is required to pay the forward payment previously agreed on, and decides whether he wants to continue working with the artist -> Artist finishes art -> Buyer pays for the art
OR
Artist opens an art shop, which has their portfolio included -> Buyer requests art, agrees with artist on details, as well as the price beforehand (forward payment as well as payment afterwards) -> Artist delivers wip -> Buyer is required to pay the forward payment previously agreed on, and decides whether he wants to continue working with the artist -> Artist finishes art -> Buyer pays for the art
If the consumer doesn't like the wip, he's still required to pay out the forward payment, but he can bail after that. Of course, he won't have to pay for the wip if it's an utter shit job, which does not have the requested features. A handy tip here is to specify in the details what kind of stage you want the wip to be in before paying out. It can be line work, but way better would be to agree on flats. If the artist doesn't deliver a wip, no problem, deal canceled.
The consumer can have several artists make wips for them at the same time. Evan have 2 open agreements after that. But, when you have 2 requests open at the same time, you can expect artists to become skeptical. Is their art really gonna be used? It doesn't matter much, a deal is a deal, and the consumer is obligated to pay out both artists. (provided there is significant improvement from the wips, and it fulfills the consumers specified details)
Portfolios are key. As an artist, it is nearly impossible to scam if you are required to show your portfolio. People will recognize your art, and will not buy from someone who has scammed before. Same goes for artists who don't deliver after the wip.
This doesn't have to be a set of rules, you can put up a tutorial in the stickies about how to not get exploited and get paid out for your work as an artist. Don't take my advice for granted, please check if I made any logical errors first, because I'm not in the art business, or even the market business at all. If I did make any mistakes, be sure to let me know.
TL;DR encourage for artists to get paid for their wips.