Locating Another User

Hi, my site allows users (buyers) to buy e-learning courses for other users (receivers). The buyer completes a form, entering in the receivers email address along with choosing an e-learning course for the receiver. These e-learning courses are presents (mostly surprises) and the majority of the time a buyer will not know if a receiver is already a member or not.

When payment is processed and the form is submitted, the site checks if the receiver is already a member, by checking if their email address is already associated with a user account, and therefore the site sends out 1 of 2 emails to the receiver:

A (receiver is already a member) - "you have received a new e-learning course from $buyer"

B (receiver is not a member) - "you have received a new e-learing course from $buyer, please click on the following link to complete your account"

Now i am thinking of adding a way so that the buyer can find / locate the receiver instead of just guessing / entering in the receiver's email address, as a receiver might use a different email address than the one the buyer knows to create their account on the site.

What is the best way to do this... i think it would be illegal to add a "search receiver by email" to the site, as this would allow users to enter in an email address and then if an email address is found, the site will auto display the user's first name and last name and username. I was then thinking maybe it is best to search for user by username... so if there is a result the site will auto display the usernames first name and last name only. Buyers can then focus on finding out the receivers username from them instead of the email address they signed up with... what do you think is the best approach... thanks in advance for your help...