Ones that make money. If you can't make money with the site yourself, why would someone else want to buy it? Why not just go into the web design business and make sites on-demand to the buyer's specifications?
good niche... so it can be monetized. popular site, it must be still trend on next 6 months, so when you sell it, it is still trend. high PR, got many traffic daily. etc good luck
All the niche can be sold depends on the personal like Just make a great design, input some quality content and work out on SEO, then you can get sales easily
People buy sites that make steady income that you can prove for at least 3-4 months back. More money your site makes the more money its worth. Also people like sites that are EASY to run.
Something that is unique. Even if the topic has done before, make sure the content and design is unique.