I'd suggest that people on the forum already have an X5, and there won't be very much response to a posting of a car For Sale. I could be wrong, but that's my gut feeling.

Not knowing where you are posting from -- which country -- I'd suggest you would get far more traffic from a site like Craig's List (free postings) or AutoTrader (fee based posting) -- there are many others. The idea is that visitors to those sites are actively looking for a car to buy, whereas visitors to the classifieds on the forum are looking for car parts that fit a car they already own. For example, I need a tow hitch for my X5 so I would look to the classifieds to see if anybody is selling one.