Try the "undercut" command to see if it fits your needs.
Scripting a fully functional mallbot is not the purpose of Mafia. However, a script can be made to add items and decrease prices periodically without feedback from the current market prices.
For instance, if you have a widget that you have priced at X, you can decrease X by Y every Z amount of time but you will be doing so blindly.
You can check your store inventory periodically to see if it is selling, this can help you guess what the best price is.
Good luck.