The seat post is extremely important on a bicycle. Using a 'bad' seat post can cause the rider a great deal of discomfort. How can a seat post be bad?
1. No or little offset. A proper bike fitting requires a saddle with proper setback. It is almost impossible for most riders to get their patella tendon directly above the ball of their foot in their power stroke without some offset. I once had a seat post with no offset and was dumbfounded as to why the bike was uncomfortable no matter which saddle I used. I expected the market would remove these problematic parts over time. Unfortunately they are still made by many manufacturer's. Do not buy a seatpost that whose clamp is not behind the seatpost.
2. Proper length. Your leg should have a slight bend when your foot is at its farthest from your hip. A short seat post will create an exaggerated bend and will cause pain just above your kneecap.
3. Get a standard diameter. Don't get anything that is less than 1" ( 25.4mm ). or anything that is wider than 35.0mm. You should be able to find most posts in this range at a local bike shop.