To build a web site like BidorBuy.co.za it would cost you around $1,500 USD. If you out-source and get it developed in India/Vietnam/Asian Countries.
DO NOT get someone local to build this web site, they will overcharge you beyond belief.
I suggest you simply do a google search for freelance work.
I think you guys are greatly underestimating this. To develop a website like bidorbuy.co.za is one thing and 1500 USD gets you really no-where. Off-shore development is always a great risk and it will take you at least 1-2 months to spec out what you want - remember that your off-shore partner will only develop to spec and you hardly ever get what you want if it is not very specific.
With any e-commerce site you will need customer support, a security team which actively scans for fraudsters, a very good payment mechanism, you will have to comply to regulatory requirements (CPA around the corner, PCI, SARS requirements as you are an intermediary accepting payments).
Most of the local sites fail as they quickly worked up a technical site without focusing on the operational support. Over and above you need dedicated, PCI-compliant hosting (as you will transmit credit card information). You will need a dedicated sysadmin team to manage your infrastructure, run security scans and ensure that you are always 1-2 steps ahead of potential fraudsters.
In an open marketplace (unlike CollectiveCow, Twangoo, Smokoo, Yourbid etc) you will also have to contend with possible trademark infringements or distribution rights (such as GHD, FIFA tickets, Power Balance bands etc) or sort out DMCA/BSA related problems (from the use of copyrighted material/images to trying to sell OEM software, modded gaming consoles etc).
Once you have done all of the above you have to still engage in marketing and Google adwords to start driving traffic through your site. In my opinion it will take 3-4 months and a few bar in the bank to start a sustainable business. One can not compare bidorbuy with sites such as Yourbid or Smokoo, as they only focus on the short-tail and their business model will probably fold within the next 12-24 months.
The hosting-cost, dedicated infrastructure, load-balancers, firewalls and bandwidth (in the case of bidorbuy in excess of 5TB per month) will cost you a few 100Ks p.m.