My thoughts:
1. Figuring out how to register without Facebook was way too tough. I had to find two links that were indistinguishable from the rest of the text.
2. What is "Brand". Is that car brand (BMW) or part brand (Bilstein)?
3. Okay, now I've gone to the "What do you want to sell" page, and I see "Brand" is the part brand (this is just because I'm poking around -- as a regular user, I'd never do this). Which makes me wonder, how do I find parts for my particular year/make/model (E46 330i, in my case)?
4. I can't do anything useful from the home page. Show me a list of brands, a search box, anything to start me on my way.
5. What does "forensics" mean in this context? The "Show forensics updates" is neat looking, but useless for someone who is buying or selling (but interesting for wasting time / kicking tires). The pictures should at least tell me what the product is.
Hopefully this hapazard feedback is useful. It looks like promising concept and you've clearly worked hard on it, but I think you need to populate it with products and then observe first-hand how people interact with it.