I think it would help to sell market apps. It's human nature to be cautious, especially on spend money, on an app when you really have no idea if it will suit your purpose. How may times have you bought software to find out it is too difficult to use or just failed to meet you expectations or preform as stated. This way you know upfront if it is a good fit or not.
Trials on a temporary test site are great if you don't spend too much time working with them; because when your done you loose all that effort and have to start over; if you decide to incorporate in you site. If you decide not to incorporate in your site, then loosing it is not going to be a big deal. Knowing the cost up front you can gauge if the product has value or not during the trial.
That said a trial period on your site gives you the option to purchase and retain you development time and data sound good to me. Maybe a way to have the app disabled after a time period with out loosing the data and development time with the option to activate it upon purchase.