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 Litecoin from the list of exchanges listed here: https://www.coingecko.com/en/coins/litecoin#markets.
You can also look at the list of all crypto exchanges tracked on CoinGecko here.