Not sure how you don't get the difference, the original owner of the phone can get the IMEI blocked at the drop of a hat, at any time throughout the phones life. The seller could have bought a used phone off ebay, decides it's not for them so sells it on MM, all goes fine, not a problem, but a week later the ebay seller maliciously, or the person the ebay seller nicked the phone off has it blocked.
I don't particularly disagree with selling phones on the MM, just don't get how you can't see the difference with phones being able to be blocked AT ANY TIME and selling a duff piece of hardware.
I think a sensible route, if one were to be considered would be to have phone sales restricted to a higher criteria of user than the normal MM, and maybe enforce having original proof of purchase or similar.
Too high risk. Same as car audio. Yes, anything can be stolen/dodgy, but both of the above are easy and common targets.
It's all great until a deal goes pear-shaped (or, more usually, several deals). Then you get a clamour of members expecting the Dons to sort it out. Like they can do anything.
Easier just to nip the problem in the bud and say no.
See what I've read later on, sure fine, you can get it blocked now or later whatever. BUT it doesn't change the fact that at the end of the day it's just a scam.
Example 1: You buy a laptop for 200pounds, you pay the money and the laptop never shows up.
Example 2: You buy a mobile phone for 200pounds, the phone comes in but it's blocked.
Example 3: You buy a mobile phone for 200pounds, the phone comes in working but week later gets blocked.
OUTPUT = THE SAME = YOU JUST GOT SCAMMED AND YOU're 200QUID SHORT AND NEITHER DONS OR ANYONE ELSE HERE CAN DO ANYTHING ABOUT IT.
So how to prevent from getting scammed ? Check some posts of the person you're buying from ( obviously if they write a lot of serious and constructive posts and been on the forums for years - I see no reason why would they scam anyone ). Check their feedback, if they have low feedback, don't buy or ask them to send it first to test it.
Also as I said if you're that sensitive about the 'phone history' part, maybe just allow if they have the original box / receipt - then you can be sure the phone hasn't been stolen.
But at the end of the day it's just another type of scam that can happen with just about anything.
At least if you get scammed with blocked phone, you can sell the bits to get some cash back ( screens - very expensive, original covers too for some ) rather than pay for a laptop/gpu/cpu/pc/ speakers, pay the money and never see them

.
So yeh, I'd prefer to get a blocked phone than a burnt CPU or pay for something and never get it.