Multiples
You'll get a better response if you say how many months worth of income you're prepared to pay per site.
For example: "If a site is earning £10 a month I'll pay £300 for it"
Then people will know if it's worth getting in touch with you.
Also, if it's the income of a site you're buying, it shouldn't really matter what the domain name extension is.
Hope this helps.
Cheers
Jaffaman.