The market for any reproduction or multi-game artwork is going to fluctuate as the interest level in the games themselves does. This level is going to rise though as the availability or production run of a multi-boardset comes on the market. People buying the boardset will likely also be the market for the artwork (unless they squirreled away some from a previous run).
When a thread or multiple threads show up with people clamoring for artwork that was produced at some point and is now scarce or non-available, that's a prime opportunity for either the person who made it before or someone else with skills to pick it up and seize on that market opportunity.
Like others have done with custom runs of artwork or control panels, offer a run with pre-payment to ensure you get a return on your investment. Then the person making them makes just a few extras for people who didn't get in on the pre-payment option and those handful will likely disappear fairly shortly. As long as the pre-payment total will cover the costs of production with a reasonable profit margin, then that's a good business move.
Of course, that's only my opinion...
