Bought an Apple Watch off someone and it won’t let me activate it as the watch is already linked to the previous owners device/phone.

The previous own needs to follow these instructions

https://support.apple.com/en-us/HT208855

The first issue you have is without the Apple ID you can’t use the Watch. Second there is nothing you can do without it. But on the plus side you have a nice bracelet. I would suggest you try and get your money back.

If you are trying to activate an  Watch and it is asking for a previous owners Apple ID and password, you have encountered the Activation Lock. This is a security feature that prevents thieves from setting up and using a stolen  Watch. You have no alternative. You must contact the previous owner to get permission to use the device. If you cannot contact the previous owner return the device to where you bought it and demand a refund. You will never be able to activate the device and no one can help you do it.