vijen6,
I did the install a couple of years ago. My memory isn't too good but here is what I recall:
1) I remember reading that I could reuse the existing (OE) metal gaskets, and that some weren't fans of the included new gaskets that came with the new exhaust. Probably overkill but I opted to buy new metal OE gaskets from Porsche. No sealant used/needed and I never had any leakage issues. Also overkill, maybe, but I bought new OE Porsche header bolts and torqued them properly and never experienced any problems.
2) I remember receiving a bunch of bolts and gaskets (more than needed) with the new exhaust and assume that is what you are seeing from your seller. I purchased new metal OE Porsche gaskets (one gasket for each side) for the header outlet to mid-pipe connection and probably used the new bolts provided by Maxspeed. No other sealant material used/needed and no leaks.
I believe these parts arrived new with gaskets on both parts and your seller just sent them to you the way he received them. When using metal OE gaskets you only need the one gasket per side.
3-4) Since I have no experience with the gaskets that came with the exhaust from Maxspeed I can't really say with any certainty about their effectiveness. But I did have second hand feedback that they weren't the best, which is why I bought new Porsche gaskets which worked perfectly. Maybe your seller had issues with the Maxspeed gaskets and had to use some type of sealant that you are seeing on the headers. It is my understanding and experience that nothing is needed except the gasket (metal) if you go the OE route. I personally wouldn't want to redo any of this work and wouldn't risk using the gasket material that came with Maxspeed system. You will probably be able to hear if you have leaks.
5) I'm not sure how bad (if at all) it would be for your engine to run it this way. Obviously without any exhaust you couldn't have any O2 sensors working correctly which would trigger CEL codes and may cause the engine to run very poorly.
My advice would be to get new metal gaskets from Porsche. Take your time. Do it once and be done with it.
I hope this helps.
Best of luck,
ddb
http://986forum.com/forums/general-discussions/21654-boxster-has-been-down-month.html