You can't sell your soul it's myth from cartoons just like devil being red and having horns and that people go to hell for doing bad or going to heaven for doing good and that one outweights the other in the end.
So to begin with you can't sell your soul since if you aren't christian then you are alredy dead and will burn in lake of fire with Satan and his angels when end comes.
But if you are Christian then you can't burn in lake of fire and are alredy forgiven from your sins past , present and the ones you will do tommorow.
How to know you are Christian ? Definition of the gospel is found by Paul in 1 Corinthians 15:1-4.
In short it's simple that Christ died for your sins and rise up from the Grave and that he fullfilled many prophecy spoken by many prophets to validate that he was the promised one to do so.
Salvation is free gift from God and you can feel secured after you beleive , maybe you alredy did but needed reminder .
Person is sealed by Holy Spirit - Ephesians 1:13-14
Free gift - Ephesians 2:8-9 , Galatians 2:16, Titus 3:5
Eternal life means it's for eternity so forever , not untill you sin next time or else it would not be eternal to begin with.
Cheer up