I purchased through the Timeshare store. They were great to work with but I was not happy with their closing company. I felt the closing company could not handle the volume from the TSS (this is just from my experience taking longer than others). But their closing costs seem to be less than other companies and no hidden fees. There are threads about Fidelity and TSS on this board, where buyers posted their purchasing timelines from offer to receiving membership number.
Have used Fidelity and DVC Resales, both are good. Timeshare Store obviously gets good feedback from others. You should feel comfortable with any of these three, but I would primarily focus on the right listing and not get tied to a particular broker.