If you are a seller, and you won't give out positive feedback until your buyer does, you suck. The buyer's job is to pay in a manner fitting your needs. To provide a correct address, and to stay in contact if necessary. If they meet these requirement satisfactorily, then you give that person POSITIVE FEEDBACK. It doesn't matter if the shipment arrives or not, they did their part. Which means, they should have their +1 long before whatever it is they purchased is even sent.
I post this because I actually had a seller who refused to give me positive feedback until I gave her positive feedback. Thus she took my +1 hostage. I told her she was wrong to do so, but she wouldn't budge. I in good conscience could do no more and the transaction went un-feedbacked.
If the Seller sends a product that isn't as promised, is it the buyer's fault? No. Take the hit. Stop sending bad product. Move on.