F I buy backlinks for my site and sell the opportunity to have a backlinks in the form of a text link from other sites to my site, cam Google tell I am doing both or either of thees task? IF so how can they tell since they do not know of the payment?
it depends on how you do it, sometimes it's so obvious that even google engine can detect it automatically the most important IMO is to link out only to related sites
It won't hurt you unless your website is a very successfull one and people start reporting you. Google does not have the time to mess with little-known sites that are not significant in their niche.