While I used to see "art sales" simply as bonus money coming in on the side, over the past few years it's become enough of an asset that it justifies an art dealer, record keeping, insurance, and taxes at the end of each year. It's currently 25% of my total income, and that has a lot of impact over my work. And just like storytelling, design and page flow--abstract principles that keep my career afloat daily--art sales also deserve to be studied, theorized, and understood.
These are guidelines, not rules. And while most of them usually work for me, they might not all work for you, so keep in mind that my market might be different than yours. Because not only do we not draw the same, we probably have different sorts of buyers.
1. Don't stay on a book for too long
I find that doing mini series of 4-12 issues is optimal for selling art. If you spend a year doing one-shots or 2-3 issue minis, you'll be hard for buyers to keep track of because it's too infrequent. And it's hard to make an impact on a title or a character with such a brief window. However, if you spend years on something like the Punisher, eventually you'll saturate your own market--people who already have your Punisher pages are less likely to buy your new Punisher pages. But if you do 5 issues of Punisher and 5 of Spider Man, the same buyer will likely want a page from both titles. A career is more stable with long-term projects, but it's not optimal for art sales.
2. 5 second panels
I remember in college hearing my professors say, "don't spend too much time on one panel. People are only going to look at it for 5 seconds." People who say things like that probably never sold a lot of art. In other words, hard work usually pays off in pages. Sure, I've seen Batman pages by name artists who can design their way around not having to draw backgrounds. And some sell for very high. But I think that pages that show patience, hard work and lots of elbow grease usually look better framed on a wall--thus they sell faster and for a higher price. The more you hand letter your street signs and avoid using Photoshop for things like copying-and-pasting panels, the better your chances of selling pages.
3. 6 month window
According to my art dealer, the general guideline is that artists will get their best prices in the first 6 months of the work being released. Within 6 months the art is still fresh, the Punisher storyline the pages portray is still a current event in the series, and buyers have a bit of time before deciding on which pages they'll pull the trigger on. After 6 month, the pages might feel stale, the Punisher storyline has moved on, and the market is saturated with hundreds of other comic pages which have stolen your buyers' attention. After the 6 month period, you'll usually end up dropping your prices. But not always.
4. Stand out
If you have a popular, unique style that's getting a lot of buzz (or a book with a lot of buzz), don't be afraid to charge more. Standing out with a unique style can make it hard to get mainstream monthly superhero titles, but it's great for selling art. Mike Mignola is a perfect example: if you want Mignola styled art, there's only one place to get it, and that's Mike. He even breaks the 6 month guideline--I'm pretty sure Hellboy prices have gone up over the years, as well as anything else he's drawn. He's the kind of artist who's so renowned that when he draws Batman pages, people are likely buying them firstly because of Mike, and secondly because of Batman. It's usually the other way around, of course. Very few artists are bigger than the characters.
5. Know your readers
I drew a Punk Rock Jesus page that included panels of Carl Sagan, Lincoln, and Galileo. It didn't have anything obvious that usually sells a PRJ page: Thomas riding his motorcycle, a sci fi background, or a polar bear. But I knew it would sell because I know my readers from meeting them at conventions, and I know a lot of them are science/history buffs like I am. So I told my dealer to charge $600 for a page he would normally asked $300 for. I think he thought I was being silly, but the page sold in a few days (I probably should have asked for more). Most people tend to think of buyers looking for 2 thing when it comes to comic art: splashes of Batman and girls with big breasts. And while those things might be true, there are many other buyers out there that will purchase "nothing really happening" pages if you know what to include (even if it's not in the script). I have buyers who are nuts for certain cars, insane backgrounds of cities, musical icons, animals, locations close to where they live, plants, you name it. If you've got a talking heads pages, I suggest drawing the characters talking in a 1960s Mustang--you'll easily get twice the money for it.
1. Using higher quality ink and paper is a positive. Markers fade over time.
2. Drawing at the 10x15 size of normal comic paper (or whatever it is) is a positive. Buyer usually don't like small pages, and HUGE pages can also be a problem.
3. Be nice to people at your table at conventions. Sounds obvious, but remember these people aren't just buying your work, they're buying you. Spend time showing them through your work but try not to pressure them too much. If you're an awesome artist (and I know you are), the pages will also sell themselves.
4. Messy pages don't always detract from the value. Mine can be pretty gross, but my buyers like them that way because they can see faded linework that didn't make it to print.
5. If you've got a ton of old stuff that hasn't sold, give it to your high-end buyers next time they make a purchase. They'll love the gift and usually come back for more.
6. Keep the contacts of your high-end buyers. If you've got something that's up their alley, they might be interested in buying some art before the book has even come out (just ask they they don't post them for a few months).
7. Having too many pages for sale on your table is bad--people get overwhelmed and can't make a decision. Better to have your choice pages out and organized in a single book, then keep the rest of it behind the table. If people want something that's not in the book, you can get it for them or invite them to cruise through the pile if they like. Having the bulk of the pages off the table keeps order.
8. Convince as many artists as possible to go digital. The less original art there is, the more money artists like us will make (if Fiona Staples had hand-drawn Saga, I would have sold a lot less PRJ pages, guaranteed).
9. Printing blueline onto the board and then inking it will usually mean lower sales. Better to use pencils (or blue pencil), and then erase completely.