I"ve just had the very unpleasant experience of having my land grabbed by a bot between me clicking to sell it (I wanted to buy for my group), and buying it back - a space of 1 second.
As I was selling it for 1L to my group it was obviously not a genuine sale, but the presumptious individual who responded to my angry IM informed me that it was my mistake.
On informing her that I'd taken a complete copy of her conversation with me in IM and in Chat, and that I had full intention of reporting the practice to Linden Lab, she eventually sold the land back to me for the same amount - funny how the bot didn't grab it when she put it up for sale.
So if you're looking to do anything with your land on mainland - beware. You may have to suffer a lecture on the evils of being angry that you've been the victim of a rip-off practice.
Personally, I don't know how Linden Labs can ethically allow this sort of practice. It means that there's no chance of anyone being able to have the chance to purchase that bargain block. No wonder real estate prices are being pushed ever upward.