There are two currencies and they do not convert into each other.

Coins

The everyday currency. A new account starts with 500. Most of the catalog is priced in Coins.

Gems

The scarce one. A handful of items are priced in Gems and nothing else buys them. New accounts start with none.

There is no way to buy Gems with real money, and there is no plan to add one that turns the catalog into a shop window. When Gems have a source it will be something you do, not something you pay for.

Selling back

You can sell an item back from your profile. You get 40% of what it cost, in the currency you paid.

That is deliberately a bad deal. A full refund would make the catalog a free dressing room, and it would make every price change free money for whoever noticed first. Think of selling back as a way to recover something from a mistake, not as a way to trade.

You cannot sell an item you are wearing. Take it off first.

What the server decides

The price of an item comes from the catalog on the server, never from your browser. Your balance only ever moves through code that rounds to whole numbers and refuses to go below zero. If you find a way around either of those, please report it in Bug Reports rather than using it; that is the kind of thing that gets fixed the same day and thanked for.

Real money

Items, Coins and Gems are traded for each other and for nothing else. Offers of cash, gift cards or goods from other platforms are removed, and repeating it costs the account. This is not negotiable and it is not about being strict: it is the single thing that attracts people who want to defraud children.