I've never made a mod though have used them a fair bit (also super excited and inspired by Original Sin & its editor to have a go myself!) and from my pov if someone wanted to sell me their mod product I would no longer view them as a modder but as a professional developer of content. I would expect a higher standard of quality assurance and customer service (information about the product for sale and post-sales support). I doubt this would be feasible for the vast majority of modding hobbyists.
I like Diacri's suggestion because to my eyes we should be looking for ways to encourage people to donate (cultivate a culture of respect, appreciation and support for modders) rather than essentially commercialising the practice.
Once you do that, it's not really modding anymore; no longer a hobby but a profession, not a community but an industry.
I also think that considering how much people love money (not saying I do ;-), some companies would have found a way to monetize mods a long time ago if this wasn't prohibitively fraught with technical, legal and practical obstacles.
