I just made a large purchase from Amazon (which has better prices anyways) so I don't need any books at the moment.
I was going to sell it on eBay, but gift cards typically sell well below their actual value. For example, it's likely that my $25 certificate will sell for around $21, subtract $2 for eBay and Paypal fees and I'm left with $19 from a $25 card. It almost makes me want to buy random crap on B&N.com so I don't lose anything...
If you don't want to sell it, the typical advice is to buy something with a rebate that you can resell for profit. B&N.com may not provide rebated options (I believe they only sell books, dvds, and cds (which are typically not rebate items)).
Uh, want to buy me a book? -D
Give it away as a Christmas present.
Buy some clearance items and sell them on Amazon shops or ebay.