The upsides of providing the proof of purchase and IMEI is obvious, the phone is sold faster and probably at a higher price.
However, I have come across quite a few sellers selling new/almost new phones - with packaging and everything - who claim that they have received the phone as a gift, and that they don't have the proof of purchase. It begs the question, is there any downside to providing the proof of purchase to the buyer?
will report to police they "lost" their phone, get police report number, send straight to phone provider for insurance, get a new phone, sell the "lost" phone at gumtree, 2-3 days phone provider will trace the IMEI of the "lost" phone and block.
PS: You can check the IMEI if the phone is stolen.