I think you can buy at flippa or whypark .. If you interested then pm me the details of what you are looking for and I can probably help you.
Do you mean websites with content, or just domains? Multiple domains can be bought at any major website like GoDaddy, 123Reh, Dynadot... Take your pick! I prefer GoDaddy personally, but they might be slightly overpriced for some extensions. But, if you're looking for websites with content, you may want to check webmaster forums (such as this) or SitePoint.. Maybe even places like Sedo if your budget is large enough.
Ebay.com , Flippa.com etc... Try all Domain Auction Sites also . Try searching google for buy Websites or Sell Websites.
Flippa, Bido, SiteDeal.org are some of the websites where you can buy/sell several domains and content websites for good money I think.