While it would be a huge pain in the butt to list all the code to this vendor system I have a free giving box located in furnation under the pyramid (by the huge sign up bulletin)
Features these vendors have
First its notecard based so you never lose any saved infomation like prices when adding objects
Second it produces no lag when not in use, passive or non active script
It has three major tracking features
Last 10 sold items
Top 10 sellers
Worst 10 sellers
The vendors i've tested up to 20 objects and i can't gaurentee stability beyond that per vendor.
I have 3 major types
Turn key
Halo
Animation
All Sources are open and the vendors are completely modifiable