Borders will only close if the EU want them to....as in punishment for us wanting to leave the EU. Tariffs will only be charged if the EU says so....for punishment for us leaving.
The EU don't want us to leave. They get too much money off us for them to replace easily. And after paying out many billions each year we buy much more from the EU than we sell to it.
Pre 1993 there were long queues at the borders and loads of paperwork ( I was talking about goods and services...not people ). That tended to slow down trade and was costly for all parties. The customs union has made trade a lot easier and trade has therefore grown enormously.
The EU has repeatedly said they don't want us to leave and the door is always open to stop Brexit.
I don't think the EU and the ideas behind it are purely monetary.
We paid in and got more out of being a member than by not being a member, which is why most MPs were remain. If that were not the case, we would have been long gone.
The people who were for leaving were people like Farage and the right of the Tory party and that wasn't just about economics. And yes, Tony Benn and his fan Corbyn were also against the Common Market, but for other reasons than nationalism.
If there is a sort of trade war, it won't be purely to punish us. It will be retaliation by one side or the other. Spiteful punishment for the sake of it won't happen. That is what people with no financial interests would want. Both sides have to live with one another and any tit for tat would be to force a deal, not to punish someone..
The EU has made it clear that a non member won't have better conditions than a member. That isn't punishment, it is justifying membership for the other members.