I Phone 3gs Help needed

Hi bought my first i phone off ebay a few month ago with working away have only been able to start the phone up turned phone on after charge dose not work at all after many hours disputing and claiming paypal ebay etc failed to get any joy so now i am £230 down the drain well at the min i am was told by a friend just to update through itunes was looking promising until i got a message with a error on saysing error 29 unknown fault have been spening too much time on google trying to look into it to try and fix issue most people have said its battery related is this true as i cant seem to find a definte answer any help would be great or info in fixing my little problem so my beloved fisr i phone dose not end up in the bin
many thanks guys nd girls