Let Lenovo confirm the defect (or non-defect) before taking further action. TS needs to follow up with Lenovo SC on this matter. I think further bickering with seller will not get u anywhere since both have their own reasoning.

Update us here whether its a defect or just a gap from the design and normal on all units. I have a 510p too from one of the seller here and it arrived sealed. It was shipped to kuching and if not mistaken, i think i noticed a small gap too.