Shared Vendor is a trigger-based library to create vendors who function like buildings/units you own. The major motivation to make this was that it seems impossible to implement a unit owned by a third party that offers Upgrades that are applied to the user who purchase them. (Feel free to comment if you know a way to do this)
Currently only Upgrades (from the Data editor) are supported, but if people want me to add support for e.g. items and unit-specific interaction (instead of player-specific interaction), feel free to let me know. :)
A quick demo of the library can be seen here: http://www.youtube.com/watch?v=aPgsDTpn3So
If you have any questions / requests, post them as a comment, PM to me or reach me on IRC (nick jorgenpt).