I sell yoga mats with a free DVD yoga session of me doing a sun and earth salutation, and I sell boxing gloves and handwraps that we use in sessions. I don’t believe I would have ever started selling anything but people just kept asking me to help them find quality products, and I got tiered of running out and getting thing one at a time. Now I have a pre-order catalog and it has worked fantastic.
My clients often ask for my “recommendation” on small equipment (e.g. dumbbells, tubes, stability balls etc.). I am more than willing to help them in that way. I don’t sell exercise equipment directly to my clients very often, but have done it on occasion. One thing to take into account in my opinion is the possibility of liability, so it’s probably worth your while to check with your liability insurer for “products coverage” if you decide to go this route.
I hope that this helps.