You cannot buy/sell directly on CoinGecko, though there will be a list of where you can buy/sell cryptocurrencies within CoinGecko.
CoinGecko is not an exchange or wallet service and we do not provide cryptocurrency buy/sell services. We are a cryptocurrency data aggregator and only provide trading information from the various cryptocurrency exchanges worldwide.
To buy/sell a particular cryptocurrency, you may look at the exchanges selling it and head to the exchange website directly.
For example, you may buy/sell Arbitrum (ARB) from the list of exchanges listed here:
You can also look at the list of all crypto exchanges tracked on CoinGecko here.