Minefield....
According to my own motor trade insurers, the only cars that need to be put onto MID are ones that have been in your possesion for 14 days and are also taxed. However, since the new tax rules came into play in October, I'm unsure how/why/if the tax side of it still matters. The road tax side of the motor trade must be another subject for another day!
I questioned my insurers a couple of yrs ago about said MID subject, and said that from the information they had given me at the time of my renewal that it was very complicated and difficlt to assertain which cars needed to be on the MID and which didn't. There was clearly some confusion on their part as one person told me to do one thing, but another in the same dept said to do another!
So, still confused, I took the decision to put every single car that is in my possesion on the MID as soon as I get them. They then stay on there until I sell them. So if I'm going to pick a car up from Coventry or wherever, I put the details on the MID just before I set off to collect it. And as soon as I've handed the keys over to new customer on completion of the sale, I go back onto MID and take it off.
This may or may not be the correct thing to do, but as I see it it's better to have everything covered in the event of an unfortunate/unlucky incident, than be unsure as to which cars need to be on the MID and which don't.
However, obviously for a small dealer like myself that only has a handfull of cars, it's fairly easy to do and once I'd been doing it it all became part of the routine. But I suppose bigger concerns with big numbers of vehicles could find it more tricky to keep on top of.
In reference to Andy's comment above regarding trade plates, trade plates only cover you for the road tax. They are nothing to do with the insurance side of things.
Hope this helps