I don't think Resource Credits should be bought and sold. I think they should be delegated to good newbies who join, and removed from newbie spammers. The reason is that it is desirable to preserve the ability of newbies to join and seamlessly transact. If there is a cost to them , they simply won't join and the blockchain will decline.

If at some point in the future, the blockchain became a super desirable place to be (eg if Ed Sheeran was a member, and his fans wanted to join), then resource credits could be allocated a value. But until then, it's a self-defeating idea to put up barriers to newbies.

What it seems to me is that resource credits are an advancement because they make transparent what was already happening before, which is that someone was paying for newbies to join. In the future, all SP holders will know how much, and have some control over the calibre of newbies whose resource credits they are funding.