The commercial district of Oasis is the only one not built around existing blackstone structure. It’s a mess and maze of tents which slowly accumulated as more people required different things. Now the selection is practically limitless depending on who you ask, what time of day it is, the seasonal rotation, among various other factors. The point is to be savvy and try to plan life around when you can actively get things.
Potions, pets, crystals, materials, basic goods (food, water, etc), wands, scrolls, everything that isn’t illegal and is a basic need of any person (adventurers need gear, the common people need food, inventors need materials, etc) is to be found during the day.
Animal (and sometimes humanoid) parts, venoms, rare materials, chips or chunks of blackstone, weapons, experimental hextech, artifacts recovered from ruins, and any other hodgepodge that is generally illegal is to be found when the sun goes down. A more interesting but far more dangerous time to buy.
When the temperature drops is the time to buy items rescued from dangerous missions outside the city. Travel is most often done when the weather is cooler, and there’s a much larger influx of items, especially ones which are illegal. Some fruits and vegetables also only occur in certain solar rotations, and understandably it takes some time to brew certain potions. Mating seasons also affect when new pets are available.