What Tzar points out is that the spread (difference between minimal selling and maximal buying price) is high, which is a sign of bad market fluidity.
There are only a few reasons for this :
- The tax, because people don't want to trade things for gold
- Rare items come in extremely short supply, most of it already in the hands of those that use them (throwing weapons are a good example). And the demand is rather low too, so few new items arrive).
- Common items (like a MW DGS) have become speculation toys, because everyone that needs one already has it.