Is it legal if I flash my phone software?

I know that is stupid question but I want sell my phone on ebay and I cant put new software by SEUS. I did it by A2 uploader and I dont know if I have to sell it or no

You can cell it nothing wrong in doing so. But if your phone was locked to some network and you have unlocked and selling it then i thing it might be against your contrat rules rest i cant say anything else

You can flash it. It is not illegal.