How can I check if a user exists?

[Richard Russo <richardr@dgx.net>]

I'm trying to sell logins to minivend via minivend, and would like it to
check if the username is available while its checking the rest of the
fields on the checkout page.

Of course there is a race condition, if two people pay for the same
username at the same time, but I assume that will not happen often
enough to warrant worrying about that yet.


Thank You,

Richard Russo
DGX InterNET