A 'stock' is a single collection of items. If Mr Patel (who only has one shop) is clearing just one line, he should write "while stock lasts".
However, if he's got several shops, each with its own stock of the product, then "while stocks last" would be better (since there are separate stocks of the product at each location).
Returning to the 'single shop' scenario, but placing multiple lines on offer, I'd prefer to see "While stocks last", since there will be different stocks of each item.
Chris