Hello, I've read that it's allowed for a single player to play on multiple accounts on the same pc. My question is, is it allowed for the same person to open up a shop in the Free Market on each of those accounts? For example, on my main account I open a shop (the automatic kind where I can leave it), and on another account I open the basic shop, both at the same time ofc.
Perfectly allowable. You can open as many shops as you want, across as many accounts as you want. You can have multiple 'hired merchants' used on a single account (this requires purchasing a hired merchant for each character.) As for the 'permits', for obvious reasons only 1 permit can be used per account (as it requires that you stay logged in.)
Also of note, in your example, you can open your hired merchant AND store permit on the same character if you wish.
Oh, good to know! Would this be the mosst efficient (as in cost efficient, for spending NX) way to circumvent the 16 slot limit on a single shop?
It wouldn't change the cost as you are still having to buy a hired merchant and shop permit, but its a lot more convenient when you don't have to vote on multiple accounts over a few days to get your shops and have your items/mesos split between multiple characters. If you get to the point where a hired merchant and store permit combined is not enough to sell all your wares, then you can think about opening shops on multi chars.