Well, basically the seller will open an Unpaid Item Dispute against you. If you still do not pay, you will get an Unpaid Item Strike.
Many if not most, sellers have automatic Blocks in place against bidder with UID strikes since we don't want to deal with flibbertigibbets who don't bother reading the entire listing before bidding.
The seller will get his eBay fees back.
You will not be able to leave feedback for the transaction.