That concession is unfortunately now almost worthless – see near the end of the thread.
Your mechanic is doing what almost everybody is doing: trying to put bread on the table at home. A €1000 Socata part puts €250 on the table at home, whereas an owner supplied part puts €0 on the table.
If you are N-reg (most of the worldwide ruddervator issue) then an FAA A&P can authorise a part by inspection.
The comment about “EASA approved supplier” is laughable. No such thing, never was. I hope he is smart enough to correctly set up a torque wrench when tightening your cylinder bolts (I would have strong doubts).
As far as I know:
Used parts require a Form 1, this can only be issued at the same time as the part is removed from its donor. So, according to EASA(and many national regulations), you can not inspect a part and install it in the same manner as you can in FAA land. This is a shame.
Let’s say that you have two identical aircraft in your hangar with the exact same instrumentation. You are allowed to remove parts during maintenance and put them back on the same individual aircraft without a Form 1. But you are not allowed to replace a part with an identical one from an identical aircraft.
Another issue with Form 1 is that it can only be issued by an organisation, so any parts removed by an independent mechanic are more or less useless for another certified aircraft. This system is a “spare parts killer” once they slip out of the system.
I’ve merged above posts into an existing thread on same topic.
@fly310 what you describe is more or less exactly the system we have for many years for EASA-reg. Used parts had to come with a Form 1. And a Form 1 can be issued (with some odd exceptions – example) only by an EASA 145 company.
There were workarounds “on the ground” but nobody ever talked about them openly. One company owner said he could transplant a part from a plane (that was my Q to him) if the plane had a CofA at the time. This applied also to removing from a crashed plane (whose CofA was not valid post-crash 
). IMHO, all this was just his imagination… @wigglyamp can post a historical rundown, I am sure.
Then we got this which was actually around for years but was kept very quiet (I’d say, predictably, not surprisingly). Now this concession is hard to use. I’d say impossible to use, if the company wants to be a bit anal.
Best approach is to do this off the books but many/most pilots have no opportunity to do that because they are in a tight grip of some company.
