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Read onlyI know it against policies to ask for personel info from buyers but what if we have to send a vedio to buyer. So can we ask for buyer whatsapp number so we can send them assembly vedio for the item they purchased from us.
No you cannot ask for their whatsapp number - read the policy on what sellers can and cannot do with buyer numbers.
What you can do is upload the video to your listing, they can then download it as required.
Don't communicate with buyers privately under any circumstances. Your sale will not be protected. And it goes against amazon policies as @Seller_hOZNPw7G8FIjl has pointed out.
I don't think there is anything wrong with providing a leaflet (QR code or link to a website) with your item so they can download the assembly instructions. Could just print it on the product package but that may not easily be seen by the customer. Use a website separate from any selling website you use so you can't be accused of diverting customers away from Amazon.
I know it is the digital age and everything is downloadable now. However I would be annoyed at having to download an assembly video to look at on a phone, tablet or computer. I would like to have them printed out in the package ready for me to assemble.
If you don't provide a printed instructions you need to be really clear on how the customer get the video to avoid returns and A-Z's.
I get it is not very environmentally friendly to provide printed instructions but I think in some cases it is better.