If there is no advertising budget, you can ask your users to advertise, thereby providing them with a discount on the following exchanges.
Any advertising should be rewarded at its true worth, and therefore the resource can conduct a referral program for its users, as well as use quite ordinary companies such as an airdrop or Bounty company, in order not only to spread information about the exchanger, but also to attract new users for which you need to come up with special bonuses and lotteries to make them want to stay.