It takes time to build a good reputation and its understandable if gamblers are hesitant to try new sportsbook/casino because of the doubt that it might be a scam. So what you can do is to prove that your platform can be trustworthy by providing a good service when it comes to security, withdrawal and online support. These are just some of the important factors that gamblers consider when they're playing. Offering bonus to try your sportsbook can also attract the gamblers and to experience playing on your site without requiring them to deposit first.
Usually new sportsbook try get trust from costumer or gambler by advertising and make promoting trough signature campaign on Bitcointalk forum, but usually have new sportsbook always give sign up bonus when early gambling platform launch. But most effective with new sportsbook get positive feedback from gambler how their website keep running well although have many people access and get withdraw process instant.
That is how business get trust, the trust come through people who is patronizing the platform and what they are reporting or saying concerning the site, with the information you gathered through peoples comment is when you will stand to build trust with them, because when some said they are scammers and with small backup as a customer you will not patronize them or place your money because you will feel that your money will get lost, so accountability is what gives trust for all this site, withdrawal process and depositing process is the key, when people testify good the trust is there already