No no no. Selling them would be doing it wrong. Going beyond level 50 is wasteful of XP and time, so I suggested the least useful way I could think of to spend the XP. To finish up, you wouldn't sell the picks. You'd use them to do something even more extravagant. Maybe destroy diamond ore with them.
Right now there is no level cap for the xp, (world edit xp orb drop and alot of time to figure that out), But i would say do two diamond picks and donate them to me