The first (and so far, only) time I saw fresh Seville oranges was at a market in France, when I was surprised by the price - the vendor was charging one euro (HK$10.50) for a single orange. They didn't look that different to the sweet, juicy specimens I'm used to except the skin was less smooth and, rather than being glossy, it was dull - but that's because most commercially grown oranges are coated with wax to make them shinier. But they felt different - the Seville orange is lighter than sweet oranges because it is less juicy and it's prized for relatively thick skin and plentiful, pectin-rich seeds.
Sevilles are also known as bitter oranges because they're mouth-puckeringly acidic.
Seville oranges are most famously made into marmalade. If you're lucky enough to find them, cut the oranges in half and juice them, reserving the seeds. Scrape out and discard most of the white pith from the now-empty skins, so all you have is the colourful zest. Cut the zest into thick or thin strips and put them into a pan. Put the seeds on a square of cheesecloth then bring up the sides to form a small bag, tie the end securely with a piece of kitchen twine and put the bag in the pan with the zest. Add enough water to cover the seeds and zest and heat until boiling. Turn down the flame and simmer the ingredients for about an hour. Strain the liquid into a bowl and reserve the zest but discard the bag of seeds. Add the orange juice, weigh the liquid and add an equal weight of sugar. Simmer the ingredients until they reach the setting point - test it by putting a small spoonful of the mixture on a chilled plate - it should be softly set, without exuding any liquid. Stir in some fresh lemon juice, then ladle into sterilised canning jars, seal with sterilised lids and process in a boiling water bath for 15 minutes.