Whatever be the reason, you'll always find someone on the web who is ready to purchase your blog (though the offering price may or may not match your expectations).
Before we look at the online marketplace where you can sell / auction your website, read these important points.
1. Buyers will always ask for site statistics and raw server logs (like unique visitors, pageviews, traffic details by country, Alexa ranks) - you can either attach these figures in the form of graphs to the auction listing itself or provide them on demand to the bidders. Just make sure that you have this data handy.
2. If you are using Adsense, Amazon Affiliates or other advertising programs to monetize your site content, be prepared to provide your revenue figures as well. Infact, most buyers will use the Adsense numbers to evaluate the potential value of your website.
3. While prospective buyers are very likely to Google you and your site before closing the deal, give honest and prompt replies to all the pre-sale queries.
4. Always get the contact details of the buyer and that includes their full postal address and phone number.
5. You are likely to face another question from buyers - "Why are you selling the site when it's earning so much revenue ?" An honest answer will help you win more trust.
6. Always specify the accepted mode of payment - it could be Paypal, Check or Money Orders, Wire Transfers or Western Union Transfers.
Where to sell your blog or website ?
Here are some of the most popular marketplaces for selling websites:
Sedo Auctions - You can list your website for sale on Sedo for free but they'll charge a commission fee of 10% when you find a buyer for your blog or website.
SitePoint Auctions - This is probably the most popular place for buying or selling established websites (that have content). The buyers are interested in sites with a decent Google PageRank, backlinks and even a revenue stream. You will have shell out a small fee to list your website on SitePoint but the bids are serious most of the times.
eBay Website Auctions - eBay has a enormous user base so your site listing will be exposed to a much wider audience. eBay auctions are generally indexed by the search engines so if your listing has detailed descriptions, expect buyers via search engines as well. The downside, you may have to deal with fake or spam bidders and ebay charges a commission from the sale.
Other sites where you can consider selling your blogs are NamePros.com and Digital Point.
Final thoughts: Do you really want to go for a public auction ? Incase the sale doesn't go through as expected, your existing subscribers / readers may not read your content with the same enthusiasm as before since they know that you are no longer interested in writing that blog.
If that's your concern as well, consider talking privately to website operators who belong to your niche. Or you may even contact the various blog networks out there. They might be interested in taking over your blog if you an established audience.