They where likely to be making excuses to save their own faces, they simply didn't know how to get it running when the battery wasn't the answer
What ever you do, DON'T put the pound shop batteries in your watches! these are almost always Alkaline batteries which last no time at all, Are often flat from the packet & invariably leak being alkaline which causes more problems. ONLY fit silver oxide batteries in a watch, they are more stable but cost considerably more!
I would think looking at that picture this requires a 370 battery. If it came to me, I'd test the circuit with a tester simular to the one already shown to diagnose any electronic fault & then run the train with a cyclonic to free any debris & then oil the jewels. 9/10 a "faulty" movement isn't it just need TLC.
Take it to someone who is a watch speicalist, rather than a counter top retailer. Most of them only know how to pop a back off & put their fingers all over a movement fitting a battery.
If none of the above applies they will then be able to give you advice & pricing on a movement.