I read a post on another forum that player-built stores in settlements stock legendary gear, however i have built many of the stores myself and have not seen one legendary item, even when the individual stores stock 30+ items.

Can anyone confirm this?

I'm level 31, I have a large Trader, Weapon and Armour store and I haven't seen any yet. I've had stores for quite some time and none have turned up. I'm not completely ruling it out but it's not looking good.

Yea i just built like 6 large weapon stores and didn't see any. The post i read reported getting 15+ legendaries per settlement.

http://www.gamefaqs.com/boards/164594-fallout-4/72832478/836764130

Yes. You need to get the level four merchants to run your stores though. These characters are specific and only found in certain areas or through random encounters. Some require your settlement to be a a specific population before it triggers.