A sensible Witcher 3 player will leave a significant proportion of the Smuggler’s Caches until they’re near the end of the game. That way, they can loot a sequence of them of them and raise ten or twenty grand in capital quite quickly – if you’re good at selling loot to the right people.
If you want multiple sets of Witcher gear (who doesn’t?) then you’ll need to loot all the Smuggler’s Caches in Skellige and sell everything off to raise the cash to do it. Just don’t do it too early.