Now with regard to inter and intra market spreads i understand that you can sell produce you dont own but when you buy back do you then have to sell it again as no one wants to take delivery.

If I understand your question then you refer to a practice called short-selling and in that case the answer to that question is no, you don't have to sell it again after you cover your short, at least not in financial markets.

so in short selling u sell then buy back and the profit u make is yours end of transaction.

Yes, that is correct.

When you go long you profit from an increase in price and when you go short you profit from a decrease in price.

Simple as that.