There could be a few reasons for this... First, type i (short for inventory).

[You are carrying 0 of 45 items] 
[You have a total weight of 3/1260]
 Nothing.

The first line tells you how many items you are carrying out of your maximum possible.

Are you at (or in some cases exceed) this limit? If so, you need to decide what you can get rid of (or what you can put in a bag).

If that's not the case, then look at the second line - total weight. Is the total weight of the objects you're carrying over your maxweight?

If it is neither of these things, then check to make sure you have enough gold (the shopkeeper usually tells you if you don't, but you can just type gold to find out).

Another reason is you may be invisible. Most of the shopkeepers won't sell to someone they can't see. To remedy this, type vis. This will, of course, end the affect of invisibility so you will have to cast it upon yourself again (or do whatever you did to be invis in the first place).