I sure wouldn't buy a "Used" ECM, there is no way of testing them before you buy.
There are companies that repair them, might google them and see what happens.
Mefi 3 is no longer avaible ..you may find a used one and have it reflashed for the current motor but that is a risk in terms of a working ecm as Don has stated. You can however use a mefi 4 and repinn your existing harness. If you respond back i have the repinn diagram and links to having it flashed to your engine. about 800 for a new 4 with a new flash
Give them a call they should have the repin info if not i will post it later. Those guys are very competent just explain the engine you have and they will send you a mefi4 with the right program in it. good luck