Diane.
I am pleased to answer you question - Private tour's (which you did not actually say you would take) are usually group tours for corporations, etc. No such thing really as an individual private tour...just an FYI. (There are exceptions and you may have found one already)
Turyol Tours usually ALL dis-embark and return to same point with out any stops in-between. No exceptions.
Very unlikely you'll be able to dis-embark in Sariyer, although I offer this option: There is one ferry that I call "Zig-Zag ferry." Starting out at either Besiktas, or Kabatas, Kadikoy you can zig-zag the Bosphorus all the way up to Sariyer where you can dis-embark. The trip will be longer as it will make several stops on the way up - still will see the same landmarks as normal tour. The ride from lower Bosphorus to Sariyer s/b about 40 minutes.
Link here for timetable in english
http://www.ido.com.tr/en/index.cfmTaksi back down and seeing landmarks is possible but no advisable during rush hour Mon-Fri 3PM - 8pm, and Sat-Sun all day traffic is murder. I assume you are considering coastal landmarks? if so - when you get off at Sariyer take any bus south with marqee that reads Besiktas or Kabatas and get off/on at each landmark. If you buy an Akbil (see my link) the whole trip back down should not cost more than 5 USD.
Even in the lightest traffic a taksi ride along the coast w/o any stops will be about 30 USD.
davcamp
http://web.me.com/dav_camp/Istanbul_Tra ... Guide.html