There are no fixed, canonical rules about selling art. Copyright laws do exist and limit the ways it can be used. But in addition, there are some generally accepted recommendations on which to base a contract.
It is very common that the buyer pays half or some other settled advance of the total price once they have seen an adequately representative version of the work in progress. I also prefer this method. In case of commissions I expect an advance of at least 50 € when taking the commission. If the final artwork doesn't live up to expectations it is possible for the commissioner to cancel and get refunded in full. In that case the commissioner accepts the fact that I'm free to offer the artwork to sell for others, even when the original commissioner's photo or live model has been used as reference.
For my part, I'm willing to negotiate on quite a case-to-case basis and the two basic pricings I describe below and use are merely guidelines. In any case, we will make a clear written contract for the benefit and protection of both parties. I'm also paying my taxes for all sold art.
These are a couple of points to consider when buying art.
• The buyer is responsible for keeping the artwork in as good condition as possible and has no right to modify it or to harm it in any manner.
• Usually the author is entitled to have the artwork in an exhibition if needed, either without or with some compensation to the owner.
My two pricing principles
• UNIQUE. There exists only one physical item, in your possession. As the author I have no right to reproduce it by painting or through any other technique to copy it or to sell prints, be it prints or digital copies, unless otherwise agreed. However, I do have the right to display photographs taken of the artwork to represent my portfolio and to advance my sales—these photos can be displayed on relevant sites in the internet as well as in catalogs and art publications. My exhibition rights are subject to negotiation.
• ORDINARY OWNERSHIP. You own the original of the artwork but not rights to sell printed or digital copies based on the artwork, unless otherwise agreed. These extended rights raise the price. As the author I have the right to paint versions of the artwork and to sell copies and prints in any format. In addition, I have the right to display photographs taken of the artwork to represent my portfolio and to advance my sales—these photos can be displayed on relevant sites in the internet as well as in catalogs and art publications. My exhibition rights are subject to negotiation.
• HIGH QUALITY PRINT. Most of my artwork can be purchased as prints on canvas or on photo quality paper, in frames or without, and also on many other products like clothing, see Merchandise. Print-on-demand companies like Zazzle, Søciety6, Threadless and Redbubble handle the orders and shipping. Note that there are many nice digital alterations of my artwork available too. You may not copy these products further for any other use except for personal home use. As the author, I own any other rights for the artwork.
All commissioned paintings from your own photograph or from a live model are always unique works by default, although the pricing reflects more the levels of my "normal" pricing. Thus you are able to purchase a commission work way cheaper.