Nigeria has a long history of prohibiting seemingly random items from being imported into the country, often to boost the country's own manufacturing industries. The list is exhaustive, but over the years it has included objects such as plastic flowers, spaghetti noodles, toothpicks, footwear, suitcases and telephone recharge cards.
One item, though, is particularly surprising: ballpoint pens and all their parts, including the refills. (It is, however, legal to bring in the pen tips, that is the nibs used in fountain pens only.)
Rather than risk finding out the ins and outs of why this rule is in place, it might be safer to just travel with a pencil.