A lot of work has gone into this and lots of nice touches, but in the end it is not about, design, navigation, good English which are normally so important with a web site. In this case its about competing with economic might!
You are in an area entirely dominated by one rival - almost a monopoly competitor - even Yahoo gave up! That makes it very, very difficult to give you positive advice.
You have to look for viable models - in the US perhaps Microsoft, Intel, Boeing. In the UK British Telecom, British Gas. All these are firms that have established a stranglehold on the market place at some time.
In the case of the two UK names Government legislation broke the markets open - by force. You don't have that level of assistance!
Boeing was only forced to share a market because three other Governments subsidised a rival. (OK, the subsidies were concealed!)
So what is your best chance?
In my opinion - imitate Apple! A unique product has enabled that company to grow & thrive - its no struggling Microsoft rival now, its expanding into a related but totally different field.
To be honest you are trying to be "all things to all men" and failing badly in that task. I went to look for the things I buy to excess on eBay - old film cameras. You have all the categories, but they are all empty! This annoys browsers (think how often you have arrived at a page that was marked "under construction" and been irritated)
I would thus list only the categories that have current auctions. Yes I know that would kill 75% of your categories, but it would leave the viable ones.
Then specialise - you can't afford to take on the might of eBay in all areas - its far, far too costly and unless you are a millionaire you would be broken by their marketing millions.
So you are left with say a unlikely and little known range of products. e.g. Matchbox labels.
Well thats not a bad thing. There are probably few venues that deal internationally and specialise in matchbox labels. Get a good name in that field and you can expand into others - say stamps & postcards - it would be an easier task then, as you now have established a viable auction and made your name to work out co-partnership deals with matchbox magazines & auction houses that specialise them. Success breeds success and you can't be King of the whole auction house pile, but a little sub-section of the marketplace will earn you good rewards.
A few examples:
Penbid
Vectis Toy Auctions
My only advice is really specialise and do it fast - before you run out of money tackling the big "E" head on!
Good luck!
Paul