Only ways he is doing this is is he is making till receipts, and works for one of the big hitters. Phones will be re-connected if you have the bill of sale, but only if a replacement has not been handed out.
Board swapping at any level.
He is at a level where it wouldn't be worth doing at one of the networks..
If he had found a way to change / mask the IMEI, then he would be smart enough to work on the development team for NASA. Again, he wouldn't bother doing this at a commercial level.
The MAX sentence for Manipulating the identity of a handset if 5 years and or a fifty thousand pounds fine.
Sadly, this is rarely (if at all) used. One of my competitors was doing this for ages to Sony Ericsson handsets and got caught. He got 2 years suspended and made to destroy his equipment..Tw@t has just been caught again doing the same thing...Profits may be up for me in a few months.
The law regarding manipulting a handsets identity extends to supplying equipment thats primary purpose is for the manipulation of a handsets identity, or supplying information on how to manipulate...Thats why most forums frown heavily upon such advice..Unwanted attention, and all.