A common trope is the sea captain who’s half a merchant, half a pirate, as occasion demands.
Such a captain might be a “fun” NPC to confound high level PCs (name level or paragon level, depending on edition).
A ship sails up to the PCs’ stronghold. The captain wants to trade.
The captain is amusing, urbane, and honest about his dishonesty. He admits he’s half a pirate (or more): and the spoils of his piracy are at the PCs’ disposal. The captain has rare magic items for sale: a few fine items that the PCs can’t easily acquire otherwise. His prices are fair. After a shared meal and a trading session, the trading ship sails off.
The ship returns irregularly, usually with interesting goods to sell. The captain will even do the PCs the occasional favor (give the PCs a ride somewhere, or attack an enemy). However, if the ship ever happens to visit while the PCs are away, and the PCs’ stronghold is weakly defended, the seafarers will sack the stronghold.
Next time the PCs encounter the captain, he will be deeply apologetic. “Just business,” he says. He’ll even offer to sell back the PCs’ stolen goods at a slightly discounted price.
The PCs might decide to fight the captain and his crew then and there, or to let him go in hopes of further profitable business. Either way, they should have strong feelings about this PC.