it is some sort of a program that pulls cell numbers from well-known online classified listings (ie KSL, craigslist, etc) and then sends a generic message expressing interest with a request that you email them.

When you do, they have your email address, which can then be sold.

I might be 100% wrong and paranoid, but that's my hunch.

Also, you can tell if it is a program/bot like the one described above if the text/email expressing interest doesn't include any specific data about your item (example: it says, "Do you still have this available?" with a link to your listing) in the body of the text.