What's so hard with giving a seller or buyer a thumbs up?
When I sell something, I often agree to a proposal, take the time to wrap it up really well, and keep the buyer up to speed untill it arrives at the buyers house. I always like to verify if it arrived and in what condition. In the past one would give a thumbs up, but nowadays after even asking to consider a thumbs up, sellers seemingly just can't be bothered. I personally find this annoying to the point of acting in the same way.
Mind you, it's not solely MG, I have the same issue with Reverb and other online marketplaces with possibilities for reviews. Which is a shame really as it sort of filters who is trustworthy and who isn't, both sellers and buyers, so you're basically helping out the community in a way.
So please consider those thumbs.