I have 5 of these at the moment. Wondering if it may be time to unload them since the price has been up and down over the last few months. What do you guys think?

I think you can start to sel 2 or 3 ...then you can wait for new movies....maybe the 3d version

In the last months I've seen that the price is decreasing

I`d wait till closer to Christmas or just after, which has been argued to be the better time to sell. Also, at this point I`d assume you`ve doubled or money on them, so you could sell 2-3 and recoup that invested capital then hold the remaining a while longer. The set has seen the bulk of it`s growth already, but honestly it`s likely to keep going up, though at a slower pace. 

Would you buy one of them, complete and in good condition second hand for 100

if you are sure it is complete and in good condition with little checking to do and instructions in good quality, possibly. 

Profit potentials isn't that high - depends where and when you will sell. 

 

I would decline. 

Pretty sure. Even the invisibility cloak is still there and the person throws in the Chamber of secrets. I have only seen the minifigs so not sure if the entire set is extra included.