Where is Majni's merchant chest?

PC

If you're on PC version, used tcl in the Console Commands. This disables collision so that the player and walk through the ground, walls, and other stuff. The player can even fly with this ability. To enable collision, type tcl again. Then find your way to the merchant's chest.

Console

The plate glitch is one of the useful and easiest ways (or not). Here's how:

This glitch works better with wooden plates and platters. Go against a wall, drop your plate, and then hold it flat against the wall. And then run towards it and when you hit it, you'll be on the other side of the wall. And then find your way to the chest. This might fail. You might need to try a few times. But this will work better with the Whirlwind Sprint shout. Just one word used could help.

One you find the merchant chest, look inside and grab all the goodies you want to steal from those traders who think they can't be robbed from you. They don't know what your doing.

Don't worry if the chest is "owned" (meaning when you hover over it, it the text is in red, telling you to steal because it's owned). You're underground. Nobody will see you from above do this, since you're beneath the ground. Nobody has X-ray vision in Skyrim.

You can wait for two days in order for the chest to reset with items. However, you need to talk to the merchant and look at the trade before you can do this. Looking at the chest after waiting two days won't work. This is so when you look at the items in the shop, the chest will generate the items that are in the shop.

I Don't Know Where The Chest Is!

Try to search underneath where the merchant is or nearby. Sometimes chests can be easy to locate, some are not.

You might need to use some more "plate glitch methods" to try to reach the chest. They are mostly under where the merchant is, as I said. Or Bethesda might be tricky and put it in an obstacle. If you're lucky, you will find the chest.

If this isn't help you either, I guess there's nothing else you can do. However, if there are more glitches being discovered by players (and being undiscovered by Bethesda who doesn't want us to do stuff like this), maybe they would help.