1. Avoid Ballot Stuffing in eBay-like Reputation Systems SIGCOMM’05 Workshops, P2P-Econ Rajat Bhattacharjee & Ashish Goel Stanford University

2. Outline Motivation System Goal System Model Inflation Resistant Deflation Resistant Discussion Conclusion

3. Motivation Reputation System is used in real world. For example, eBay, Amazon, and Epinions. Ballot Stuffing  A seller colludes with other buyers to undertake fake transactions in order to higher his reputation Bad Mouthing  A seller is targeted by a group of buyer to lower his reputation

4. System Goal Restrictions on reputation premiums  If the reputation premium is inflation resistant (or deflation resistant), it will lower incentive for a seller to indulge in Ballot Stuffing (or Bad Mouthing). Reputation premium  Incremental cost a buyer willing to pay for dealing with a seller of higher reputation

5. System Model (1/3) Intrinsic error rate ε  A seller commits errors due to things not in the hand of him. All errors result in negative feedback. Ballot Stuffing rate ρ  The rate at which fake transactions leading to a positive feedback (for a particular seller) Perceived error rate ε p  The rate at which the system observes a seller is making error is denoted by ε (epsilon) ρ (rho) ε p (epsilon p)

6. System Model (2/3) Reputation premium ξ  Incremental cost a buyer willing to pay for dealing with a seller of higher reputation A function of perceived error rate ε p Assume reputation premium decreases with an increase in perceived error rate ε p Transaction Cost τ  A fixed cost which seller must pay to the central system for each transaction ξ (xi) τ (tau) ε p (epsilon p)

7. System Model (3/3) Transaction cost per genuine transaction ψ (psi)  ψ(ρ) = τ(1+ρ) τ ： transaction cost ρ ： ballot stuffing rate

8. Inflation Resistant (1/2) A reputation premium is inflation resistant if the following relation holds In other words, gain from reputation premium will decrease due to faster increase in per genuine transaction cost.  If you want to increase reputation premium, you have to pay more cost. ξ ： reputation premium ψ ： per genuine transaction cost

9. Inflation Resistant (2/2) A reputation premium is inflation resistant if Linear Reputation Premium where Perceived error rate ( x ) Reputation premium (ξ) τ ： transaction cost ρ ： ballot stuffing rate ε p ： perceived error rate ε ： intrinsic error rate

10. Deflation Resistant (1/2) β  Fixed transaction cost for buyers η  Fake transaction rate leading to negative feedback χ  All transaction cost the buyer intending to deflate the seller’s reputation. χ(η) = βη

11. Deflation Resistant (2/2) A reputation premium is deflation resistant if the following relation holds In the hope of decreasing a seller’s profit, the malicious buyer must pay more cost than what the seller loses. ξ ： reputation premium ψ ： per genuine seller’s transaction cost χ ： Total buyer’s transaction cost

12. Discussion – A caveat In linear reputation premium, a ρ-initial strategizing seller is one who does ρ k positive fake transaction and then undertakes k genuine transaction If, a ρ-initial strategizing can benefit.

13. Conclusion Need special treatment to new sellers Transaction cost is important in avoiding Ballot Stuffing and Bad Mouthing. If detection mechanism is provided, the problem in caveat will be released.