Anthony, as far as I know if a seller has a -4 feedback their account is automatically suspended. As others have said you can always leave appropriate feedback, however negative may not be the right choice after all you say you ordered 3 but 1 of them was defective, So in my opinion a neutral would be the best way to go.
Also tell your seller that under the Sale of goods act 1974, he is responsible for the delivery and effectiveness of the product, and also that you are entitled to a refund/replacement of the defective product.
Take a look at this link
http://www.consumerrightsexpert.co.uk/FaultyGoods.html
Look out for this article at the bottom:
Second-hand, Sales and Auctions