flippa is the best, followed by namepros and digitalpoint forum for the domain, sedo is one of the best
You can definitely find buyers to your dating site, but other things will factor in that equation to determine if it's worth buying. How old is the domain? How much traffic is the site currently getting?, Is the site currently profitable? Can you verify your traffic sources etc. Just a few things to consider....
Here on DP, and also the Warrior forum. However, not sure they're the absolute "best" places for this business...
Flippa is the best place to sell your website and you can sell your website on popular forums like DigitalPoint etc
To be honest the best luck I have had is putting an ad on the site itself. It's how I have sold all of my sites. I know that for some businesses it's not considered the done thing to show that a site is for sale, but it worked in my industry. And a site that GoDaddy valued at $80 on their valuation tool actually got me $10,000 Lol!
If you're just going for selling the domain then I'd say sedo.com, afternic.com, snapnames.com. If you're including the content I'd go with flippa.com.
I found http://www.flipfilter.com very usefull (checks DP, Flippa and other sites to see what is for sale)
Flippa for websites Sedo for domain names There are many forums like DP with Marketplace. You may try selling the sites or domains on forums too. But, I doubt if they could be sold for as high price as on Flippa/Sedo on the forums.