There are a lot of non-playable characters (npc) that sells items, and they are not marked on the map like the blacksmith, or the ingredients seller in the city, but many of them sell some useful things that can come in handy, especially if you don't have enough materials to cook some food, you can buy one ready to go for a quick food buff. Note that some npcs here don't show up at all times. In case they don't show up when you get there, try to use the "wait" feature, from the main menu just below the mail icon, and change the in-game time to daylight.
Starting in Mondstadt, you can go to the tavern to buy a couple of drinks. These still provide some decent buffs in case you need them, and they are also at an affordable price:
Wolfhook Juice x2
Berry & Mint Burst x2
Apple Cider Vinegar x2
The next one is not far from Mondstadt, in Springvale, the place that involved Xiangli when you first met her. The lady near the cooking spot appears during the day and sells some food:
Chicken-Mushroom Skewer x10
Northern Smoked Chicken x2
Close to Brook, there is also an npc that sells meats, useful for cooking certain dishes and you're feeling lazy to farm them:
Raw Meat x10
There are several npc's that sells useful items in Liyue, and many aren't shown on the map. You can start by going to the teleport waypoint on the right in the city.
Going left from the teleport waypoint has an npc that sells some irons:
Iron Chunk x10
White Iron Chunk x10
Noctilucous Jade x5
To the right of the teleport waypoint:
Delicious Grilled Tiger Fish x10
Stir-Fried Filet x10
Going further down the road and you will see another npc with a food stall:
Delicious Zhongyuan Chop Suey x2
Mora Meat x10
2 npc's that sells fish near the edge of the harbor on the right side of the city shown on the picture above:
Fish x5 (left npc)
Crab x5 (left npc)
Shrimp Meat x5 (left npc)
Fish x5 (right npc)
Fish x3 (right npc)
On the left side of the city, you can go to the herbalist to buy a recipe and some materials.
Recipe: Lotus Seed and Bird Egg soup.
Last one worth mentioning is the one in Qingce village. Go to the teleport waypoint and jump down to the side and you will see Ms. Bai: