Technically you are correct. If there is no 'consideration' given for an announcement of any kind, it is legal. With that said, we know FCC rules are written in a way that allows them to be 'interpreted'. You can pass on one level and not on another. I've found it is better not to push the envelope too much...it tends to make them think you might be trying to get away with something, making them want to look closer so they can find something. When you're close to that line, the human tendency is to think there must be something wrong somewhere, making the person set off on a quest to uncover that 'wrong'. If you're above the line on other things, you likely wil not be looked at with that much scrutiny.
I recall a problem one non-comm had with an underwriter. It seems the Underwriter was an attorney and the phrase 'XX years at the same location' raised red flags (xx was something like 25). The FCC said this was a ploy to give this attorney an unfair advantage by implying this attorney was more experienced and thus, better, because his office had been located at the same place for so many years. The phrase was purely legal to say in an underwriter's announcement but under scrutiny, the FCC figured the unfair advantage angle, fining the station. To cite another: it is legal to include a logo or identifying statement that helps identify a business. Walmart's logo is 'Low prices every day'. Technically it is legal to include this in the underwriters announcement but this is easily illegal as it is 'competative' giving an unfair advantage to the underwriter. We need to look at the whole picture.
I think the rules as written are unnecessary in many respects with all the specific verbage and such but we're stuck following these.
My advice is gingerly approach the Tradio idea on the air, treating it as an underwriting announcement and I doubt you'd have any issues. If you let a caller say "We're moving and the new place is too small for our Washer and Dryer, so I gotta move it today. It's a practially new Maytag, works great and if you can get it out of here today, I'll take $300 cash", then I think you might find the FCC frowning upon this. Sure, the caller might not have offered any 'consideration' but I think the FCC could come up with a way you benefit or breached the rules.
While considering this, there once was the question of a 'Commercial Club' (sort of a Chamber of Commerce) that wanted to offer free commercials for every business in town on their non-comm. The idea was in synch with their stated mission, 'to promote the business community and encourage local shopping habits'. The businesses were to offer no consideration whatsoever to the station or the club. The club has no membership fees. Yes, we're talking price and item advertising here. By the 'consideration' standpoint, this would be legal. By the mission statement of the non-profit to promote local businesses and the economic health of their community, this would seem legal. Is this not the same? Sure it is a for profit business that gains from the freebie, but isn't the individual on Tradio trying to gain financially and aren't many businesses owned and operated by an individual? Would the FCC claim the $300 price giving the man an unfair advantage over others that might the same model of Maytag washer for sale?
An option I would think might pass muster is more generalized promotion. For example, a non-profit with a non-comm ran a station with a Visitors Information format. This was a full power non-comm FM. They mentioned locations of every gas station, all restaurants and some details on menu items, including hours and spoke of shops that might be of interest to those motoring down the freeway through town. This was mingled with local attractions, etc. There was no consideration, technically, for any announcement and these announcements were presented as a 'matter of fact' versus being commercial in nature. For example, they might mention all the fast food places in one announcement. This was legal. Text might be "If you will be stopping for gas, our town has several options right along the interstate for easy access. At exit 312, motorists will find Shell gas with a 24 hour convenience store, a Mobil station with convenience store open 24/7 and Chevron gas and diesel at the Carters Truck Stop, open 24 hours with convenience store, restaurant and shower stalls. Coming from the south at exit 304...".
Another consideration, if the non-comm is a 501(c)3 is how the IRS would view this. They could very well be more troublesome than the FCC.
I don't have the definitive answer here, but I think logic and reasoning must be applied here. I am not an attorney and did not play one on TV, nor did I spend the night at a Holiday Inn. The information is worth just what you paid for it. I simply worked for a guy for many years that was a banker, CPA and knew how to befuddle the IRS. You might say he rubbbed off on me.