The actual Good Friday Agreement says very little about cross-border trade

George Carty (on Twitter) trotted out a lazy misconception: "Surely the main problem is the Irish border (again) - IIRC we can't be in EFTA and also (as required by the GFA) in a customs union with the EU."

Fraggles points out in the comments: "The GFA does not state that NI must be in a customs union with the EU. It simply doesn't. Who keeps making this stuff up?"
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In such situations, I find it useful to write down what you know, and look up what you don't.
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1. Fraggles is quite correct. The full official text of the Belfast Agreement (as it was originally called) is available here. It's 35 pages long so I haven't read it all, but if you search for the word "border" it shows up ten times, but only as part of the catch-all expression "all-island and cross-border [matters]"; the word "trade" appears once as part of the expression "trade union".

Fact is, the GFA had very little to do with cross-border trade. Somebody who worked for the Northern Ireland Office (or Department or Ministry or whatever it was called at the time) under John Major (Tony Blair picked up the baton and finished off the process after he became PM) confirmed that the main aim was ending The Troubles, duh!

They did this quite simply by stuffing the mouths of the leaders of extremist movements/terrorist organisations on both sides with gold. The UK governments' own bullet points are:

* the creation of a democratically elected Assembly
* the creation of a North/South Ministerial Council
* the creation of a British-Irish Council and the British-Irish Governmental Conference

In other words, creating lots of lovely, well-paid public sector/political sinecures which were awarded to the likes of Gerry Adams and Rev. Ian Paisley etc. Every man has his price. In return, said trouble makers agreed to lay down their arms, and all in all, it has worked very well (at considerable cost to the UK taxpayer, but worth it IMHO).
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2. We know perfectly well that there was only a 'hard border' between NI and RoI because the NI police needed to minimise the amount of weapons being smuggled into NI and/or terrorists moving back and forth.

(Ever since Irish independence, the UK has always honoured freedom of Irish citizens to move to and from the UK (and vote in elections here), and there was never a reason to try and impede cross-border trade.)

The GFA enabled the border to be 'softened' to the point of irrelevance by buying off the terrorists/extremists. The only reason the UK would reimpose it is a resurgence of nationalist terrorism.

So people who say that if there's a No Deal Brexit we'll go back to the Bad Old Days of hour long queues at the NI-RoI border with armed guards and frequent inspections have completely lost the plot.
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3. If you search the full text for the word "EU", the most relevant paragraph is this:

The British Irish Council will exchange information, discuss, consult and use best endeavours to reach agreement on co-operation on matters of mutual interest within the competence of the relevant Administrations. Suitable issues for early discussion in the BIC could include transport links, agricultural issues, environmental issues, cultural issues, health issues, education issues and approaches to EU issues.

I think that Brexit falls squarely under their competence, so what is the British Irish Council doing about it? I haven't heard a peep from them. In fact, I'd never heard of them at all.
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4. More facts:

The population of NI voted by a reasonable majority to Remain in 2016.

The vote share of the two biggest Unionist (broadly, pro-Leave) parties in the 2017 General Election was 46%, against 40% for the two biggest Nationalist (broadly, pro-Remain) parties.

So a bit of an uncomfortable contradiction there!
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5. We know that borders are artificial, but a line has to be drawn somewhere and compromises made.

The UK is not a single political unit, there is a whole hodge-podge of territories which are 'British' for some purposes but not others (Isle of Man, Channel Islands - both use GBP and have same titular head of state - Gibraltar, Falkland Islands etc). Scotland is autonomous in some respects and has a fairly powerful Assembly (which they cheerfully refer to as the Scottish Government), Northern Ireland has a similar Assembly, which spends most of the time trying to get itself shut down again. And we manage just fine.

Similarly, there isn't actually a neat line round the Member States of the EU. There is a whole hodge-podge of territories which are in the EU for some purposes and not in the EU for other purposes, full list here. Gibraltar is on this list as well. And they manage just fine.

I'm thinking, surely it is not that difficult to think up some fudged arrangement whereby Northern Ireland falls into the categories of half-in, half-out of the UK and similarly half-in, half-out of the EU?

Where there's a will there's away. Problem is there is absolutely no will on EU side to do something sensible and Mrs T May is being held to ransom by the fairly extremist DUP.
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6. Cross-border trade is very important to the NI and RoI economies, making up about 5% of either side's GDP. I'm not sure if cross-border workers are included in that.

But NI or RoI trade with Great Britain (i.e. the rest of UK) is several times greater than trade between themselves.

Which stands to reason. Although the island of GB is a bit further away from RoI than NI is, the population of GB is thirty times the population of NI and fifteen times the population of RoI.