If bought through classifieds then it will be a private sale, and presumably second hand.
You then don't have a supplier to refer to for warranty/after sales.
If you know details of the original shop you can refer to them, but they are notm obliged to support you.
Your next stop is the manufacturer, if the original customer did the 'right thing' and sent back their warranty card then they can tell that your current details are not right, (if they actually cross refer). They are not obliged to provide warranty to subsequent customers.
After saying all that the shop staff, and stall staff out on games are nice people and are happy to help you out with problems.
But taking the mick and trying to get them to fix your brand new super flash USA import for free can be pushing it a bit to be greedy. If you can't be bothered to support your local dealers then you can't expect them to oblige you.
But if you do want the new gear thats not available here then you can ask for help and be grateful. Try to be a good customer locally too, buy their paint etc, be friendly when popping into the stall and get yourself remembered as a customer before you break your marker and expect them to fix it.