I agree with tomozj here. Adding one command to instantly sell all of your properties isn't very realistic and by doing that you're basically taking out
the biggest role-play feature Las Venturas Playground has. However, I believe players should have an option to either sell of their properties instantly using "/sell all", or alternatively, go round selling their properties individually. Choosing the latter
should be more beneficial to their bank account, too.
"/sell all" obviously wouldn't spam the chat with property new-messages, rather one message, "[BB]Phyc0 has sold of his/her properties!". Selling the properties individually wouldn't cause spam either as most of the 'big' properties are spread out all over the map.
By doing this you're fixing two problems; first being the obvious spam, and secondly, Las Venturas Playground's economy. Because let's face it, most of us are probably going to use the "/sell all" option anyway.
All of the above has already been mentioned in this topic, yes, but I believe this is the way to go.