Thanks for your reply!
Well, let's say I wanted to buy other's gig for real and left a review and he/she did the same and bought my service what makes us getting banned for this ? I need their service and they needs mine I guess it won't harm.
I don't think it will be so easy to fool the feedback mechanism. It will raise an alarm. Instead, pay someone who has history of purchase in fiverr and ask him/her to order a gig. Avoid feedback from brand new accounts. Never exchange feedback.
As for your logic, it's their business and have their own algorithm, not necessary they have to abide by your logic to survive, rather its you who have to follow their rules if you wan to survive.