I'm not sure Propstore will be a good guide of value in this case, they don't usually sell well at Propstore as it's not their main market, and they have the expensive additional fees. The market is a bit weird right now, a lot of people will say right now that things don't sell well, but at the same time, bigger, rarer items will still sell for good money, even privately and we might not know it. In any case, if you are determined to sell the MPP, you can always start at 3.5-4K and lower your price over time if it doesn't sell.
If I'm being honest, I think in a case like this, reputation will be important though, people will have a hard time here buying a 4k item from someone that just registered, they will have an easier time paying that much money to someone with a very well established reputation, which is completely normal of course. I guess in cases like this, this is where Propstore or other well established selling house come in handy.