Question
 

If i buy a second hand license, does it 'expire' meaning after a certain amount of time i cant use it?

Currently all licenses sold (VB4 licenses) never expire, you will always be allowed to run any 1 copy of vBulletin up to 4.9.9 (or whatever the max is) forever.

And the same goes for 3.X, it doesnt 'expire' so i can use it forever?

If it was a "Leased" license then no and is in fact expired however if you had a fully "Owned" 3.x license then yes indefinitely.

Leased Licenses actually can't be transferred. So, if you buy a second hand 3.x license, it will have an expiration date, however that is just for which version you may download. The license will allow you to run that version (or any version released prior to that version) on a website until the end of the internet.