Dme and Ecm

Hello, I was advised by my mechanic that my DME computer is no good. He told me they are not programeable so its not worth it to buy a second hand one. So checked on line and prices for new ones were outrageous. So i checked Ebay and found some. Some I contacted the seller and he assured me it was plug and play no programing necesary. So against my mechanics wishes I bought the part. My mechanic installed it and low and behold it doesnt work. Does anyone out the have any expierience with DME and ECM? Thank you!