BAD SELLERS GET BAD RATINGS. Don't feel like you're obligated to leave good feedback! If you're not completly satisfied with the transaction, FUCK 'EM.
Although that is true, the other person you are doing buisness with has the right to return the favour. (which in the end can make both people look bad) I have done buisness through ebay and had to wait 4 months for a stupid computer chip to finially reach my house (the seller was located in Canada. So it should have reached my door step in 2 weeks not 4 months) I sent him a money order Express post which made it to his house in 2 days according to the Canadapost tracker, and he claimed payment was not recieved quick enough, so in the end be both got negative ratings. But finially after I contacted Ebay and explained the whole story to them, they agreed to erase the negative feedback all together. However even though I did end up winning that one, it took almost a month before my feedback rating was changed back to normal, and one seller on ebay wouldent sell to me because of that.