I know there are already plugins out of this, but they are all inactive, I just hope that someone could take over the build!
I have requested a plugin on the description you are saying. I remember i saw one not to late ago. I will try to find the plugin for you
http://dev.bukkit.org/server-mods/simple-region-market/ This works (Personally tested and verified) with 1.2.4 dev builds and allows you to both sell and rent worldguard regions. If you click the link to his website and go to download you will see a button for the latest version.
as mentioned above simple region market does exactly what you want and is very much so active. i have the devloper on skype. its actuallly a very very user friendly way of doing it. you make a sign which once made will be displayed somthing like [for sale] (region name) (Price) 30 x 30 x 30 Or if you want to just rent the plot to them for a certain lenght of time. [for rent] (name of person renting it, and how long is left (it counts down) 30 x 30 x 30 iv now encorperated the renting system as opposed to buying. if you select the price say as £1000 per day, and set the multiply x letting option to for example 30. a user can click on that one sighn up too 30 times and rent it for 30 days, for 30 x's the price. and if at any point they wish to top it back up, once its started to run down by at least one day. they can re click it! i used to sell plots, but im now only going to rent them, this way inactive players plot will automaticallly come back up for rent with time! hope that helps...
I've used it on my test server and in conjunction with using worldguard flags, this functions EXTREMELY well. It is currently one of my favorite plugins and one that has worked like it was advertised from the start.