As above, dealer stamps are now becoming a less of a priority in service history, esp. when dodgy sellers can hold of stamps.
Specialists are becoming more popular as most are owned usually by ex-manufacturer employees.
Or as I do, DIY. I service mine myself using the parts from dealers, keeping all the receipts for proof of service history and mark it in the service booklet. There is also a guide to what is done on each type of service so you can check what needs doing. The prices of parts from BMW are good they are about the same as the auto factors and sometimes cheaper.
Fuel filter for a 328 is £15 from BMW, from Halfords it is £20!!
Its mainly labour costs at dealers. I changed the rear trailing arm bush on mine for £8, BMW wanted £180 per side!! A good specialist will be about £100 per side. Its always going to be cheaper doing the DIY route, but a specialist will be cheaper (in most cases, so check) than the dealer and I should think will have no detrimental effect when selling on.
Edited by dave 328