We have had this up several times.
I suggest you ask the seller for help.
If the seller cannot help you, it's probably because he never tried fitting the belts himself, and
in that case there's a good chance that what you bought won't fit your deck.
Sorry, but that seems to be quite common. I have a fairly large old jam jar in my repairshop stuffed with Ebay belts I have
taken out of units coming in for repairs here, because "things didn't work out".

Some (attractively cheap) asian off-the-shelf standard-size belts are very close to the size of the belts used by B&O. But other parameters like elasticity and friction etc. are typically far out.
This matters in particular where delicate sinter bronze bearings are used, - in motors and the finer workings = almost anywhere in B&O.
Spindles run in a thin oil film inside the bearings, practically without friction and wear. If you introduce a too high
sideways pull (read: fit a too tight belt), the motor will run rough as the spindle treads through the oil film and starts to run metal to metal.
This reduces the lifespan of the bearings considerably - often taking it from several decades or even centuries to a few weeks - perhaps only hours.
Typical symptoms are that you suddenly hear screeching sounds or "rough rattling" from the deck for a couple of minutes and then it all stops.

I have seen sewer pipe gaskets (!) sold on Ebay as belts for Beogram 1000. I was baffled when I found out.
The size looked about right, but the elasticity was way out. It was hard, almost like plastic, and no good at all.

Just last week I had a Beocord on the bench with a new type of belt (some silicone-based material I think with something else added in) - new to
me at least.
The owner told me, that the deck ran fine for the first two-three weeks with the new belt, then slowed down and ground to a halt.
The belt material didn't agree with the idler pulley plastic material, and they had eaten away at eachother until it all was one sticky mess.
Good cleaning, new pulley, new belt, new motor and a repair to the voltage regulator in the power supply.

Belts are not "elastics" or "rubber bands". There's so much more to it than size and looks.

I can supply correct belts and I can also include mounting instructions (photos and english text).
Correct parts are not THAT much more expensive.
I can also supply new turntable belts and new rubber parts for the idler and the reel tables for your Beocenter.

Martin