In the next few days Cameron will come back claiming a great deal and urging us to vote to stay in the EU in the referendum due in June.
If he doesn't, I'll be amazed.

No one is actually setting out what will happen in the event of a vote to leave. Will it kill trade and costs jobs?

I personally don't think so and here's why.

In 1999 the UK had a trade deficit of 11.2 Billion with the EU. By the end of 2014 it was 61.6 Billion.
Exports to the EU have averaged growth of 3.4% over the same period and imports by 4.7%.
Exports to non-EU have grown by 6.5% over the same period and imports 5.5%.

So we have a trade surplus with the Non-EU.

If the UK leaves the EU there is a 2 year negotiation period in which we can agree trading terms between the EU and UK.

What happens in those 2 years is anyone's guess and much of it will depend on who is handling the UK side of the negotiation.
Hopefully not Cameron as he's already shown he's not that capable in my opinion.

But let's assume we cannot agree and so the default position of WTO terms and tariffs is in place.

That means then that what we sell in to the EU will be subject to import duty as it arrives there as will what they sell in to the UK.

The "UK Losers" (and of course there would be some) in terms of trade would be companies that sell to the EU as there delivered price for their goods would be 10% (for example) higher than they are now.

The "UK winners" would be two types.
The companies who sell domestically and compete with EU companies for business in the UK.
The companies who sell international to non-EU as the UK would then be in a position to negotiate trade deals independently.

There would be a benefit for our Corporate Tax as well. Companies such as Google would not be able to offshore their business in the same way they do now. They would most likely need real UK presence and therefore be subject to the taxes as any other UK company.

There would also be an additional duty benefit. So long as we have a trade deficit with the EU, we would also receive more in import duty than the EU would gain in export duty.

Of course it is a very complex picture as if duty was in place, the amount and nature of our trade to the EU would change, but based on where we are now the financial benefits in the short term would be huge (and I haven't even factored in the EU payments we make).

Which is why I think the EU would want to continue with a free trade agreement, just as we have now.
The EU, which is struggling for any growth, cannot afford not to trade with the UK and it cannot afford for it's exports to the UK to be effectively 10% higher than they are now, therefore losing business to UK business and non EU business.

Without doubt there are wider issues - it isn't just about trade. We have the agriculture and fisheries policies, the environmental policies.
But the UK for some reason always seemed to end up on the wrong end of all of these so I do struggle to find negative issues arising from a Brexit.