FYI, here’s the excerpt from tumblr Community Guidelines:
Username and URL Abuse. Tumblr’s URLs (usernames) are for the use and enjoyment of our users. Don’t squat, hoard, collect, trade, or sell Tumblr URLs, and don’t register a URL for the purpose of impersonating an individual, organization, or brand.
So you’re being a douchebag by hoarding urls and you expect me to trade you mine. I don’t think so.