What is the ruling on online currency trading, given that the selling process involves placing the currency on a waiting list until another member buys it, with the currency being immediately deducted from the seller's balance, and the possibility of a refund at any moment, and the site charging fees, while purchasing is done immediately from the currencies on offer?

The intermediary website is considered an agent for the seller or for both parties in the currency exchange contract, and agency in currency exchange is permissible by consensus. The قبض (seizure/possession) occurs through immediate transfer between the website and the buyer, or between the accounts of the seller and the buyer when the exchange takes place. There is no harm in the website taking a fee for its agency.