New to bitcoin core I don’t know where my balance is after selling a digital item

Short Answer

You probably need to wait for Bitcoin core to finish “synchronising”. This can take days or weeks depending on your computer and your Internet connection.

Longer Answer

I got the money when it was done pending

In the Bitcoin network, “pending” is not an adjective used about money. A bitcoin transaction can be “unconfirmed”. I guess the word “pending” might be used by Roblox about a Robux transaction and maybe this causes confusion?

it went to the exact wallet address …

All is well if you mean a Bitcoin transaction, with a known Bitcoin Transaction-ID, paid money in BTC to a Bitcoin address controlled exclusively by your Bitcoin-core Wallet.

… and completed

The adjective “completed” is not used for Bitcoin transactions, perhaps that is a word used by Roblox about their internal money. Bitcoin transactions change from “unconfirmed” to “confirmed” and accumulate an ever greater number of confirmations as time goes on. A bitcoin transaction is confirmed when it is included in a new block added to the top of the Bitcoin blockchain. Every block added after that, linked back to the block containing a transaction, increases the count of confirmations of that transaction. Six or more confirmations is generally regarded as safe.

I never got it when using bitcoin core

Bitcoin wallets do not contain money. They do not really receive money. All Bitcoin money is recorded in the list of transactions called the blockchain.

I think it best to think of a wallet as a key-safe, as a window on the blockchain and as enabling you to create and circulate “unconfirmed” transactions.

As a window on the blockchain, Bitcoin core must collect a copy of all Bitcoin transactions since the beginning before it can sum the credits and debits to calculate a balance. Credits are transaction outputs paying to an address your wallet controls (has the key for). Debits are transaction inputs spending prior outputs for addresses your wallet controls.

The process of collecting all transaction data in the blockchain is called synchronising.