I got around 2K TC. I tried buying something in the market for 400 TC, but it said I didn't have enough. Why?

What did you try buying?

You most likely don't have the Qi. Qi is awarded for every game played. Some items have Qi restrictions.

yup, before buying you should check how many qi you have. Some items got qi restrictions.
like pure items (you need to be 10th dan - 10000qi - to use them).