please help!! the engine starts but shuts off after 2 seconds

[big zach]

i am buying a used 2000 328ci from a dealership. it has 139000 miles on it .it has an s tittle and needs a wind shield, hood, L. headlight, and L. fender. The salesman said it will start but shuts off after like 2 seconds. He thinks its the encoding of the key or the security system while another salesmen thinks its the fuel pump... he wants 3500-3700 for it

Any ideas why it wont run?