eBay suggest that you request the contact details of your buyer/seller if you've been unable to make contact after three business days. As far as I know, there's no provision for you to default on the purchase just because the seller hasn't contacted you within the suggested timeframe.
Under normal circumstances, a non-responsive seller is as frustrating as a non-responsive buyer; however, it doesn't appear that you've attempted to make contact with your seller. Although it's generally considered good practice for the seller to initiate contact with the buyer, it's incumbent upon both of you to make contact. As such, you're equally culpable in this instance.
There's no requirement for you to pay the seller's fees when you renege on a bid. After a specified period of time has elapsed, the seller can apply to eBay for a refund of their final value fees. This is a frustrating process for the seller however, and results in a strike against the buyer's account, which can ultimately lead to suspension. For this reason, and to avoid negative feedback, non-paying bidders will sometimes offer to pay the seller's fees.
In this case, I think the right thing would be for you to contact the seller, explain that you no longer require the item, apologise, and offer to pay his or her fees. Going forward, you might want to read the eBay User Agreement, which clearly prohibits persons under the age of 18 from using the site.