Just kidding, it sounds like you want to sell a software or web project. I'm not aware of a marketplace specifically for what you need.
Has your product already been marketed in any way? Have you already found a demand for it? Posting it for sale on the official website and any channels where you've found a market for it would be a start.
The only other advice I can suggest at this point is to find where a potential buyer for your product might hang out online. Join those communities and actively participate before seeking a sale. Once the community knows and trusts you, you might be able to generate some interest for selling your product.
There's isn't really an online marketplace for businesses. My advice is to make a list of:
Contact them (individually) and ask them whether they'd be interested in considering the purchase of your business. Put together a summary of your business (a couple of pages) and send to the interested parties. Try to include a ball-park price: it will make it easier for them to consider.
If you get multiple offers, then you engage with them to pick the best one. If not, you might get some feedback about why they don't think it's a good deal for them. You might also get an idea of other people to sell it to.
Best of luck!
There were a couple of Stanford students that wanted to sell their code, and couldn't find a buyer. they ended up staying with their business, and making it grow. If you haven't guessed it yet, I am talking about Larry Page and Sergey Brin, the founders of Google. Selling isn't always the best course of action (though a product as good as theirs is rare)
I am not aware of a market where you can offer your readymade code for sale... Usually people order code to their specifications. If there is a demand for such a market, it might be worthwhile to build it. EBay started as a place to trade collectibles because the founder's girl friend didn't know where to buy the pez dispensers she was collecting.
If you can be more specific regarding what your code does, I might have more specific advice. Otherwise all I can offer is EBay and craigslist.
BizBuySell is an online marketplace for business. I haven't used them personally, but they may work you and others in your circumstance.
You didn't mention if your product has customers - if so, I think you're much more likely to find a buyer. I understand that with SaaS types of applications a buyer will often consider the worth of the business to be 3-12 months of its revenue, depending on their risk comfort level.