Thanks for this post. I myself have a small app that I am thinking whether to publish as GPL or other licenses (MIT or Apache). But I am worried about some shady users who would rather distribute the binaries without the source.

I would react the same. At least, before other users change my own code or even profit from it, they should try to contact me. But MIT license does not oblige users to disclose the source. Nothing against MIT license, but it's somehow not a good fit for stuff that don't require the full source in order to run.

Why not release a new version, and publish it as GPL? That would be clearly a violation if they accept bids on Elance like that. Or try dual licensing.