Board Thread:Roblox help and discussion/@comment-5903643-20180415150513/@comment-35364724-20180421095939

It's not necessary illegal per se. If you're found to own or participate in one of those sites, your account would be immediately terminated. In Roblox's Terms of Service, it specifically states under the heading " Final Payment ":

"All payments for Robux are final and not refundable or exchangeable, except as required by applicable law. You may not transfer, assign, sell, gift, exchange, trade, convert, lease, sublicense, rent, or distribute Robux except through the Service and as expressly permitted by us. Any disposition or attempted disposition of Robux in violation of these Terms will be void and will result in immediate termination of your Account and your license to use Robux. We do not recognize or condone any third-party services that may be used to sell, exchange, transfer, or otherwise dispose of Robux. We do not assume any responsibility for, and will not support, such transactions."