white harbor

Given that Manderly wanted to mint new coins for the independent North, the sheer tonnage of silver that would have to be purchased and then imported is not only ridiculous, it'd be an absurd proposition- especially as a long-term financial decision with no mentioned investment from House Stark or any of the other major Northern Houses. Besides, where would they be getting it from if not from their demesne? The Northern Mountains? Transporting that much silver that far, and on foot, would be expensive as hell. Not to mention guarding it. The Westerlands? The transports would have to either sail past the Iron Islands (bad move) or go around Dorne. Braavos? I don't recall anything saying that Braavos has any silver mines. 

Other options are spending silver to buy silver to then re-mint that silver which would result in a massive net loss of silver. Trade? Yeeeah, I could see this one but it would turn into a minting battle between the North and the rest of Westeros; each time a trade ship comes into port, they'd take each other's silver and sent it to the minthouse.  

IIRC Manderly also gilds spearheads with silver for some guards too. That's a pretty stupid flex if you're importing, even if you are wealthy like the Manderlys.

All in all, a respectful disagreement. I think they'd have to have at least a few, to support minting new currency and gilding spearheads with silver.

Unless I'm missing a quote, this is false. 

"As windy as he was vast, he began by asking Winterfell to confirm the new customs officers he had appointed for White Harbor. The old ones had been holding back silver for King’s Landing rather than paying it over to the new King in the North." - Bran II, ACoK.

This suggests only that the old customs officers were swindling Winterfell/ Robb in favour of Joffrey, nothing speaks of import.

Customs Officers aren't limited to import either- they control the export of goods and prevent banned items from exiting a nation. Tariffs/ customs duties can be levied on exported goods too, although in our modern times it's largely fallen out of favour. From Britannica.com "Export duties were common in the past, however, and were significant elements of mercantilist trade policies. Their main function was to safeguard domestic supplies rather than to raise revenue. Export duties were first introduced in England by a statute in 1275 that imposed them on hides and wool. By the middle of the 17th century, the list of commodities subject to export duties had increased to include more than 200 articles."

Withholding/ taxing the export of silver (and other goods) so as to maintain it's value is a strategy as old as time. So it's a possibility that there is, in fact, customs duties levied on specific goods one of which is silver.