That is a very interesting problem you have to consider.
Researchers are notoriously short funded. So it is not surprising they want the world for very little money. You have to decide if this is a project that is worth your time and effort. Including the time and effort to set up and manage a sales system if you are going to try to extrace a fair value over time.
First, do you trust them to remember to come buy the image from your site instead of just asking a colleague to borrow their copy that was bought, just for this powerpoint thing they need by tomorrow...
Might you be bertter off getting them to leasing the whole collection for a yearly payment? (lease to own!). You want to minimize the hassle of this whole experience for yourself and for them. If they know they will get additional content over a several year peiod, they are then locked in to staying the course and feeling like they get more value for each yearly payment. And you get more time to fit in the project in your spare time when you are free of better paying work. If they can't pay reasonably up front, you trade for longer schedules and reasonable payment over several budget cycles. They can sell it to the administration like they justify software licence fees.
If you were to set up a sales site, I am thinking something like Square Space with all the modules they offer might help minimize the hassle of setting up and maintaining something. But you are investing your time and money, it is not a free business model. And would you limit sales to just this group or are you now a free agent selling thier ideas to the world?
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