i need to sell my 8gb iPhone 3G and i'm trying to do what it says in the thread title 
the only thing that seems to work is downgrading it to 3.1.3. I still get the 1015 error when the restore is complete, but I can use TinyUmbrella to kick it out of the restore and it then works absolutely fine.
If I try to then update it to 4.2.1 then I get the 1015 error and I can't get it out of the restore loop no matter what. I try downgrading to 4.1 and get an error code of 14 and it doesn't even manage to get a quarter of the way through the restore cycle.
Been trying dozens of different ways since this afternoon but i'm just wondering if I just need to accept defeat and put it on ebay advising that it's been previously jailbroken and happily works on 3.1.3 but nothing else ?

the only thing that seems to work is downgrading it to 3.1.3. I still get the 1015 error when the restore is complete, but I can use TinyUmbrella to kick it out of the restore and it then works absolutely fine.
If I try to then update it to 4.2.1 then I get the 1015 error and I can't get it out of the restore loop no matter what. I try downgrading to 4.1 and get an error code of 14 and it doesn't even manage to get a quarter of the way through the restore cycle.
Been trying dozens of different ways since this afternoon but i'm just wondering if I just need to accept defeat and put it on ebay advising that it's been previously jailbroken and happily works on 3.1.3 but nothing else ?