Question about a tactic some people seem to use on cryptsy

[Lrt]

Note: I'm not trying to pump or dump any coins, I'm asking about how something is done.

On a few coins over time I've noticed that a huge sell wall seems to disappear when you try to buy it.

Is there a way traders can put coins to sell then pull back the coins when someone tries to buy them? It has happened with a few very low cap coins and there is one coin in particular that it seems to be the case with.