Hello and welcome to the creator community. As the first seller of a 3rd party accessory I may have at least a couple answers to your questions.
You’re right that there is not a dedicated section of the forum for custom parts that aren’t expansion cards, expansion bay parts, or the input modules. I don’t think that this is any kind of intentional way of keeping out other projects. I haven’t seen anyone’s project get kicked off the forum for not fitting into one of these categories. The various cyberdeck projects would seem to disprove that. If you haven’t already seen it, please check out the List of company or individually driven projects.
It has been going well despite some hiccups at the beginning with setting up a website to accept orders and payments. But I sorted that out pretty quickly. Small payment processor things do occasionally show up from time to time, but I’ve found customers always very accommodating as long as I’ve been transparent about the issues and timely in my communication.
There were a couple people who half-joked about me selling “closed source” (though I do sell the design files) products for an “open source” device. But most people have been understanding that design, manufacturing, and shipping takes time and money which should be compensated for and incentivized monetarily. I would say that although most people here are tech savvy, the number of people that have chosen to buy my 3D printed drawers is higher than you might think compared to buying the files and printing them themselves (74% bought physical).
I have found good reception, yes. I will put a caveat on that though. Although the Framework customers (your potential customers) are very passionate about the product and tinkering etc. There just aren’t very many of them. First of all, you are only getting a subset of potential customers here on the forum, maybe what 12% of all framework customers? Maybe @nrp could chime in on the proportion of customers that come to the forum.
Of those forum users, you then get a subset that wants to buy anything third party. Of them, some subset that would be interested in buying your product. In my 4+ years of selling SNACK drawers, I have only sold to 103 customers. I am very grateful for each and every one of them, but I think I have just barely now made back the price of the printer, plastic, and website upkeep. Now, maybe I am undercharging, but increases in prices might not have actually resulted in more money due to lost customers.
So, figuring out how to set pricing, getting customer attention outside of the forum and cutting production costs are going to be challenges to overcome.