Hi,
If you are looking for the clone of ebay or amazon then the development of them will cost you a tons.,
So First I need to know the exact what you are looking for.
Like from your description, I got that you are looking for a multi vendor website where a person can sell the things and buy as well.
Please let me know when can we discuss it in detail.
I have 7+ years of exp and I have a huge portfolio as well.
REgards:-
AshishM