The key would be getting targeted traffic to the site. The search "online personal training" gets 27,100 searches a month in google. If you can build an easy to navigate site with a few tools and a good sales page
you could make some serious money with this type of business. The main key ..... getting your website on page one of google for the search "online personal training".

As far as cashflow it would be up to you to put together a few example packages that could be purchased via paypal however the majority of the business would come from custom packages designed by you
for your potential clients. I would suggest doing a few for free to have positive testimonials or reviews for your services. After you get things going you could look into paring up with a supplements company
to recommend to your customers as another additional way of bringing in cash, bodybuilding.com pays 25% commission.

Good luck