[Huz-Z]

Careful. What you are describing sounds just like what other folks have reported when the power top motor has failed altogether. IE, it unlatches, makes the usual sounds but does not open / close.

I think Shipkiller may have a DIY for this on his site to install a new motor. Its not a cheap part, sadly. For price haggling with the car's current owner, call a local BMW dealer and get a quote on having a new top motor supplied and installed, and deduct that from the asking price.