I am pretty sure this is NOT petrified wood. It looks more like fossil coral. Whenever people find something and they ask "what can I sell it for?" I pretty much answer the same thing: not much. Most common fossils are not worth anything but a few bucks and sadly your piece is no exception. I'd argue that you keep it on your desk or coffee table as a conversation piece. If you tell me what city you found it in, I can give you a date range.