It's pretty good of the seller to send you a new battery and lens! As ic-racer said, the battery is probably the cause.
The 6008 is a pretty good camera - I use it as my point-and-shoot

- but it does have some areas of weakness if they aren't serviced.
Battery is probably the first thing to check - charge it up and see how much charge the camera thinks it has. If it dies quickly the cells might need replacing (easily bought of e*ay and installed with a bit of fiddling, or get a local battery shop to do it). Once the cells were replaced my camera never seems to run out of power.
Otherwise clean all the contacts (battery, lens, film back, shutter ring). If that doesn't work you might need a refund from the seller (although the Xenotar is a nice lens).
Keep us informed on what you find. It's always good knowing what can go wrong and what the solution was.