What happens if I don't pay my paypal invoice on a received item?

Legally you are stealing. The seller has the option to report the incident to ebay and paypal will torture you until the payment is made ( phone calls, ruin your credit, take you to court - oh yes they do this). The seller can also report you to the police for theft. Look, the seller did the right thing by you and now you need to step up and do the right thing by them. Resell the item if you no longer want it.