Your 1st step is to reach out to the seller and express your dissatisfaction to see how you can reach an agreement. Give the seller a chance to be the stand up guy you thought him to be. This should have been your 1st step before putting this post out there.
Pending what the seller says then you can then decide how you want to react and if negative feedback is appropriate.
If the seller isn't reasonable then we all should know and appropriate feedback is fair and warranted.