I had an experience with a seller recently, who was harrassing me into using the Star Ratings for "their" benefit, so I think it is important that buyers understand what the stars are for... They are there for two things.
1) To let buyers know how good a sellers service is.
2) To let a seller know which parts of their service are below an acceptable standard to the buyers.
The stars are primarily for the benefit of the buyer!
The only way the buyers can get an honest assessment of the seller is if buyers rate honestly. If you think you got a 4 star or less service, then rate it as such. The seller will tell you they will lose benefits from frequent 4 star or less ratings. Well that's just tough, if that's the case, then those sellers should be making the effort to provide a 5 star service, not harrassing buyers into giving falsely high ratings. If you buy something from a seller who charges £5 for P&P and it comes in a jiffy bag with 40p stamps on it, then that is unacceptable and you should rate it as such.
For the sake of other buyers, use the Star Ratings honestly, they are for YOUR benefit.