I don't really understand how to sell things on here. I figured out how to list it, but I have no idea how to know what price to put on it? How do you determine what something is worth when pricing it? I'm just confused and trying to figure this game out haha, thanks for any help!!

When you go to list an item, there should be a pop up with information like this displayed in the box(I'm using what came up for one of the items in my inventory as an example):
Item Information for:

[Mushroom Fairy's Outfit] x1

Current Listings: 0

Average Price: 0
Lowest Price: 0
Highest Price: 0

Sales in Past 90 Days: 3

Average Price: 358,667
Lowest Price: 36,000
Highest Price: 520,000

If your item is currently one listed for sale, it'll say the current listing prices it has. Based on the prices listed(Lowest, Average, and Highest), you could use them as a guideline to determine how much you want to sell the item for. Such as listing for the same price as the Lowest currently for sale, the Average of all of that listed item, or the Highest price of that item currently listed.

If your item isn't currently for sale but has been on sale in the last 90 days, you could use the prices listed in the last 90 days to determine the price you want to sell the item for. These prices reflect what the item has previously been sold for. Highest being the highest price it was bought for, and lowest being the lowest price it was bought for* and on average*All previous sales in the last 90 days determining the average price.

And if an item just hasn't been on sale currently or in the last 90 days, there of course won't be any price to go by, sadly.

There's also an Item Marketplace(Clickable Link) forum, where you can advertise the items you want to sell.
If you don't feel comfortable selling on the Trade Broker, you could list in your forum post the items you want to sell, and ask other users who are more familiar with pricing, what they may suggest selling the item for.
You could also look through the Item Marketplace forum and see what other users have for sale, so that maybe if you have the same item and want to sell it, you could maybe use their price as a guide on how to price your item.

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Sorry if this isn't the most helpful information, but I do hope it helps at least a bit.