Little will be accomplished by a non-watchmaker opening the back, as the problem would be unlikely visible to the naked eye. The slot in the middle is for the battery, the back comes off using a special key which would grip the 14-sided back. Best advice has come from Shum already, get in touch with a specialist. Go to the Electric Watches site and you'll see why this advice makes sense.
That watch is already almost opened. You see the numbers 1 and 0 on the back that is how little one should unscrew the back then one puts a sharp blade between the case and back and gently pop it out and often a fingernail is enough to get it out. When you screw it back on again the 0 should be at the crown position there the 1 is now. But taking the back off is just to see how it looks and care should be taken not to touch the balance.
If the seller doesn't know how one opens it don't tell him as it only shows that he doesn't know enough about watches anyway.
I did some dumb things when I started out with vintage watches but luckily I didn't have the chance to get inside a nice watch like this and do it harm.
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