> is it streaigh forward to register purchased plugins to a second hand desk?
When buying so-called add-ons, it makes no difference whether you buy them for a new or used SQ.
You only need the serial number of the hardware and a DNA number of the “software”.
You will receive an activation code based on these informations.
> Also is there any way to demo the plugins before purchasing?
Something like that would be ideal and fair, but unfortunately you have to buy a pig in a poke.
The only way I know of is to try the add-ons on a console where they are already installed.
You should also consider:
If your SQ breaks seriously and you don’t want to repair it, your add-ons are gone too.