I was talking to someone the other day who was merging two pieces of land that they had for sale and when she merged them, someone swooped in and purchased them at the price they were listed at. That price was the price of one of the pieces of land. I've seen postings of this happenining before, but I think I have a solution to it. Someone may have come up with it before, but I haven't seen it.
I think that when you merge two pieces of land where either one or both are currently for sale, one of three things should happen.
1) A pop-up comes up warning you that the land will go up for sale immediately for x amount of Lindens and be immediately available to purchase. (easy to implement)
2) Lands that are up for sale can't be merged until they are both marked not for sale and a popup notifies you of this necessity. (fairly easy to implement)
3) When merging, the price of the resulting pieces of land is additive, meaning if one piece of land is 3,000L and the other is 12,000L, the resulting plot will be listed at 15,000L. (seems fairly easy to implement)
I mention the last one because that is what the person I spoke to thought would happen, and I think that it's a reasonable addition that will hurt nobody, but will prevent the exploitation of the ignorance of new users. Sure they learn after one time, but this is a much better solution.