I think it is realistic, but without seeing the numbers to crunch it is hard to see if it is the best decision or not?
Maybe a donation level package?
If you donated $X per year then you get to place X number of ads per month
donate the next highest level and get to place more ads per month?
I can see it being a PITA for businesses to pay for each ad, but a one time donation fee that allows them more ability to post is good. May be good to poll some of the bigger market place users and see what they think of this? I think the real offenders are probably 3 different people that I could name off the top of my head and most of them are not really businesses as much as flippers making a quick buck (or trying to?).
I am guessing most genuine businesses actually update their donation on regular basis.
On that note, I need to reup for personal support. Incoming soon.