The second sentence is correct. The verb pity needs an object after it. Who are the people that you pity? The first sentence doesn't give us this piece of information, the second one does. You pity those people who buy things from the person you are talking to.
Here are a few examples.
• I pity the people who have to work in this weather.
• Rama pitied Bharath because he hadn't completed his assignment.
• She took pity on the boy next door.
COURTESY : The Hindu (The National News-Paper) - India