Your right to a full refund (rather than to a repair or replacement), in respect of defective goods, lasts only up until the point where you're deemed to have legally 'accepted' the goods. If the case had arrived in apparently good condition, and the clasp had broken some time later, you could only expect a repair or replacement. However, it's clear that you've never 'accepted' the case and therefore, as you state, your right to a full refund remains intact. (The trader should also refund both the initial carriages costs and the cost of returning the case to him).
If the guy is operating a business online (irrespective of whether it's directly or through eBay) you shouldn't have had to email him to obtain an address for returns. It's illegal, in the UK, for anyone to trade online without providing a returns address at the point of sale.
If you experience problems with this trader, you can find the contact details for your local Trading Standards office by entering your postcode here:
http://www.tradingstandards.gov.uk/
Alternatively (or as well), of course, you could refer the matter to eBay's disputes procedures.
Chris