This guide only works if you are not in phasing zone like Orgrimmar, Stormwind etc...
Requirement : Server Hop, Cross Realm Assist...
If you wanna buy for example limited items more then once - for example choppers for bike you just need to do server hop, and "new" vendor will spawn, and you can buy items from him again.
I discovered today this because i had need for some "limited items .
Good luck in your search with this knowledge now!