I have designed a few of these online marketplaces and I can tell you from experience that in this day and age, phone numbers are a deterrent on a signup form. I advise against it and agree more with the cons.

However, that does not mean that you cannot get feedback from students. Form the past websites that I have built with student demographics, I have found that they like to interact with each other, so a better way to get feedback is to set up a forum in your online marketplace. Also, just adding a comment thread like blogs have is a very good way to get feedback, even more effective than phone numbers.

Also, one reliable method that I found works is an incentive based feedback survey when students can earn pennies on the dollar to take surveys. If you go this route, you could offer 25 cents or $1 or even $5 to the first 20 or 100 students that fill out the survey answering all your questions that you would normally have asked over the phone.

I hope my suggestions are helpful to you. I can always assist you in building your online marketplace. I have more suggestions for you as well.

Bruce