As to only allowing people who've given some support to ETS to sell, if a donation is required to do something, it's no longer a donation. Could that potentially get ETS in trouble?
I have many years experience with 501(c)(3) corporations, I was treasurer and president of the Fire Company and I've been intimately involved in the startup of a few.
Basically, a donation that conveys a level of "membership" in a non-profit is a straightforward thing. "Benefits of Membership" are defined by the organization, and can range from access to information that is not public to access to events to special access to places that are "off limits" to non-members. For example, I am a member of the Lehigh Valley Zoo, which gives me special access rights to the park, gains me entrance to events and more. We belong to a number of non-profit organizations, and they all offer SOME form of "member benefit" even if it's just a newsletter.
In cases where gifts are given in exchange for membership (sign up, get a mug) there are specific dollar amount limits and other rules pertaining to the "in-kind" gift returned to the donor. So, no, donations in exchange for a "members-only" area of the site where there are different rules are not an issues.
Obviously, this is just a discussion, and nothing here is actionable or represents a decision by ETS, Inc. in any way, but I'd be interested in assessing the interest level for a concept like this, and wondering what price point would be acceptable (if any).