Id be a little worried buying that Xeon for the ebay link, its a very low price, and a few feedback suggest the seller is selling fake intel.

I give up.

You cannot sell "fake" Intel because there's no company in the world that can reproduce/manufacture a fake processor, and the seller has all the positive feedback you could want. This is called an engineering sample and since you seem not to know anything about computers, you might want to look into what things are and why prices are lower before dismissing them. This is the best deal you can find. Up to you if you listen or not.

A 'fake' CPU is usually a cheaper model modified to appear to be a more expensive model. Of course no one has the ability to actually manufacture a knock-off CPU that works.

It's not appearing to be anything that it isnt. It appears to be a E5-4617 C1 revision CPU (engineering sample) and it has all the same specifications as a C2 which is retail. The difference is, this one ought to have been destroyed after testing/manufacturing but it wasn't. Since their sale is "not legal" you cannot really get 1600+ € that the retail version is worth. But this processor has quad QPI that works (so you could potentially build a quad socket system made of these), it does not have hyperthreading (but retail version doesn't have it either) and some of the features might have been slightly downgraded (like turbo boost going to 3.1 GHz instead of 3.2) but it's MORE than worth the money. Especially since you could buy two of these for the price of one i7-3930K and have the same performance (single E5-4617 vs. i7-3930K @ stock frequency).

If you are worried about the ebay link why not just get the i7-3820 (sandy bridge E) 4 memory channels 3.6 Ghz, LGA 2011. Having the other two cores is nice if this is your only computer but if it is soley a computing machine then you should be fine running 4 threads on it. If you are going to be doing post processing, matlab, wasting time at work on youtube on it then yeah you might need the other two cores...

Do you think the mainboard will support the CPU? It is not in the list (the e5-4617 cannot be found in any CPU-support list). Which mainboard offcially supports this processor?

Sure, the CPU is supported by the quad-CPU boards. I would like to know, if single CPU boards (with X79 chipset) support this CPU?

They will. They do. It's just that no one tested them so they didn't end up on any QVL list because why would anyone need a CPU that's built to work in pairs or in quad mode in a single socket. However, the CPU is the same as any i7 socket 2011, it just has quad QPI links..

I know for a fact it works in several ASRock Socket 2011 boards and ASUS P9X79 and P9X79 Pro/WS.

Many thanks.

One more question.
Are there any overclocking options for the XEON? The CPU is locked, whereas the i7 can be overclocked a bit. Comparing the performance of both, I would expect that 4-cores at 4.4GHz (i7-3820) should give a similar result as 6-cores at 2.9GHz (e5-4617)?