iPhone acting strange then never turned on again.

Hi, i have an ihone 3gs bought on ebay. worked great for aproximately 4 months. never fell, no water has fallen over it. for no apparent reason the back light started flashing nonstop, so it sucked up the battery, when connected to power, phone worked just great. as soon as disconnected started flashing again. after a while it didnn't charge or wouldnt be recognized if connected to the computer. i turned it off to restart it and see if the problem would go away but it never turned back on. i thought it was the battery so I ordered a replacement one and changed it. phone turned on and worked perfectly withought the flashing light but when connected to the power source the screen blanked and turned off. i decided to leave it charging for a while to see if battery needed to be charged and the phone just got really really hot, after that it has never turned on again.

i am maybe thinking it may be the dock connector, or even the motherboard?? if so, are these replacement parts readily available?

PLEASE HELP!!!!!!!!!