As were the Romans before them.

"The Romans also traded for slaves. We know, for example, that a great portion of the Roman wine sold in Gaul was paid for in human currency (as many as 15,000 a year). One aspect of Gallic and German slavery that the Romans found interesting was the willingness [at least from the Roman point of view] of individuals to sell themselves into slavery to pay off their own debts. European tribes also sold their war captives to Roman slave traders and merchants. Finally, the practice of piracy provided a steady supply of slaves. Pirates would routinely kidnap individuals from seized ships and sell them into slavery. Similarly, they could attack coastal towns and villages and sell the population wholesale into slavery. Finally, they often worked with gangs based on the mainland. The gangs would attack and seize the towns, turn them over to the pirates who would arrange the sale of captives in ports with slave merchants, and split the profits with the gangs."

http://abacus.bates.edu/~mimber/Rciv/slavery.htm