If I understand your question, you have a WLIP that you want to sell (at least) and you want to know about settings and data.
As far as I know, the data would be in your hard drive under C:\Weatherlink. If you delete everything there your data, settings, etc. will be gone.
However, any data already uploaded to Davis via WLIP (and consequently CWOP/WU) is probably "permanent" on the various databases.
I don't think that anything that would would sell associated with the WLIP (or rest of the station) would be a problem. Unless you put some custom IP addresses into the WLIP settings that you want to protect, I wouldn't worry about it. If there is something stored, you could use the web interface for the WLIP and set it back to DHCP. If you are going to sell it, you probably want it on DHCP anyways so that you won't get questions about the address from the buyer.
Greg H.