Selly is just a platform and there is nothing wrong with it , I am using it over 2 years now and always fast support and solution whenever any bug or problem arise . Selly is one of the best and trusted digital item selling platform in my opinion , the scam you mention its the scammers who open autobuy store and scam people that do not mean selly scammer , Selly is free to all to sign up and use and they also have Premium plan with some extra perk .
eBay has tons of scam sellers that don't mean eBay is scammer lol .
If meet scammer on selly.gg, how to appeal to get coins back?
You can't basically , for that you have to do due diligence while you buying . Check the seller's reputation and past history and feedbacks and all . Crypto never can be charged back . Selly support can help you if payment made for PayPal .