In order to upgrade the weapon and armor shops' level so they sell better stuff, you need to sell to them Materials (note that the item shops are based on your Achievements). In order to get these Materials, you can find these as drops, or you need to break ♥♥♥♥♥♥ or Blunt equipment, that is getting their durability to 0. As you keep getting higher tiered equipment later on, it may prove difficult to get Materials, so here's a guide showing a quick and easy way to get these, especially early on.

First, go to Firstbridge.


Then, on the upper left is a seperate screen where you can fight 3 Rockits. Be sure to not wear anything you care about as these enemies may quickly corrode them.


Kill them until you get the Pervy Liquid (if you haven't gotten any, simply exit the screen and come back so they respawn).


It's worth noting that while you move around, you can find ♥♥♥♥♥♥ equipment in the random chests lying around, which is a plus, but we have a less random strategy.

Next, find Sukumizu who is right by the start, on the lower right side.


Now use the Pervy Liquid on her (she will not aggro on you).


She drops a ♥♥♥♥♥♥ Robe, pick it up and wear it, and while you're at it, wear whichever ♥♥♥♥♥♥ weapon you happen to have.


Now wearing these abominations, you may go back to these Rockits, or as I prefer since it's closeby, the ones behind the waterfall. You don't need to kill them.


Just get covered in their slime balls attack until it breaks your equipment.


You finally get the Materials. Now, since you most likely have leftover Pervy Liquid, go back to Sukumizu and recover her in the sweet nectar, she'll drop another ♥♥♥♥♥♥ Robe since you left the screen. Repeat the process of breaking your equipment.

After a while, you should have a bunch of Materials. Now go to your city of choice, then go to the Weapon/Armor shop. There'll be a message box saying if you want to sell your Materials, like the one below. Choose [Hand it over].


Now the shop/s should've upgraded, if not you may have to do it again. Note that, in the case of Totory, since the Weapon and Armor shops are seperate, selling your Materials in either of them will up the level of both of them.

And... that should be it. This should allow you to buy some good equipment that will make the earlier parts of the game a breeze, in my case the Hardcore Club. Now you have to do it multiple times, and I think the maximum level is 5, but at least you can upgrade them quickly should you use this trick.