They are redoing the way PBE accounts work and wiping most current ones. Riot is picking a handful of helpful PBE accounts to not get wiped, but most old ones are.
New ones will be wiped each round of testing unless they are ones that provide a lot of feedback.
New Signup Page:
Announcement from Riot:
TL;DR - Selling PBE accounts is no longer feasible. Was fun while it lasted.