On February 6th, in this feature request I said:
A control in the Edit->Land that you own where you can set said land For Sale, and put a price on it. [...] If these features make it in, let it be known that I intend to open the first SLentury 21 office =)
So I'm getting a late start, considering the feature has been there for a while now... but if nobody else is already doing it (I don't want to step on any toes here), then I will.
What I'm thinking is something similar to the Ad display kiosks near the welcome area. If you have land for sale, give me a notecard with a snapshot of the area, a landmark to it, your asking price, and any other info you want to include. The real estate "office" would be located in my tower in Welsh, and this will be a no fee service. That's right, no cost to the lister, no cost to the prospective purchasers.