Do people buy/sell tickets before they are issued in July? And how do you sell a (blue) parking pass on there?

There's a few on there already. I've never had reason to sell my tickets previously so don't know about demand - will they be easy to shift?

With certain tickets (including Green Man) you can list them before you have the tickets in hand. If it sells out (and it has the last few years) then you'll have no problem selling the tickets.

Regarding the Blue Car Parking pass, I sold one last year and you just have to start a Green Man sale and when you get the drop down menu asking what sort of ticket you want to sell, just select 'other' and go from there.