I am selling my iMac over Facebook and a potential customer is asking for serial number. I am not really comfortable with the fact to share with him but do not know what will happen if I do or why he wants it? Any are downside of sharing iMac serial number? Should I ask for upfront before sharing?
The buyer could also just want to make sure that it is an authentic product- Apple Support- Check Coverage.
As long as the iMac is registered to you, I don't know of any problem sharing serial numbers.
I've purchased several Apple devices on eBay (not Facebook) and I always ask for the serial number so I can verify the warranty and purchase/activation date with Apple. I would say that a serious buyer for an expensive item will always ask for the serial number.
In fact, adding the serial number could increase your price because buyers will be assured that you're not selling a stolen item.
The normal rules apply here so if you think the buyer is shady, new on Facebook, or gives you the creeps, then follow your intuition and don't provide it.
Any are downside of sharing iMac serial number?
Many years ago, various third-party software was bundled with new Macs. It is still possible to obtain an upgrade discount on some apps by providing a valid serial number of a relevant Mac.
It's possible that some other 'proof-of-ownership' fraud could be perpetrated.
While this may not affect you directly, it does so in the sense that the effects of such things do have a cost to us all.