This addon was developed a long time ago for community called ELGCommunity that has since closed. The website can still be found here

The addon allows players to purchase adverts from an NPC for a cost to in-game currency that would display on the screens of all players connected to the server on the gamemode DarkRP within a game called Garry's Mod The message cost would depend on the length of time shown and the number of characters.

Within the file lua/buyadvert/sh/sh_ba_config.lua you will find the configuration to adjust within BuyAdvert.

If you wish to adjust the code and use this on your own server, or copy to help learn Lua, feel free!