I will give tips point by point as following:
Never buy from anyone that approaches you on the street.
is the seller bluffing or do they really know the bike’s history?
Do they look like they could be the owner of the bike they are selling?
Examine the bike thoroughly to check its condition. Also look to see if it has been security marked. You also can check the frame number Bike Register ID by using Bike checker facility on Bike register database to make sure that the bike is not listed as stolen.
is the seller can produce an original purchase receipt and service history, that’s a good sign of genuine sale.
I think this tips will be helpful for second hand bike buyers.