No experience with them, but a long-time user of keh, so that’s my frame of reference. I went to the mpb site you linked and noodled around a while to make my own comparison for an ongoing gear search.
Not saying mpb is bad, just making comparisons. Their site is pretty clunky to use compared to keh, in particular their search filters, in the category I examined at least. Their selection was also lots smaller.
As I said, not saying that mpb is bad at all. Just didn’t suit my needs or tastes. Someone else will have to contribute actual experience with sales and purchase. But I will say that keh has been kind of the gold standard for condition rating and warranties in used equipment for decades, and it’s certainly worth checking them as well as mpb before you buy.