I have all of my domain names re-direct to the page that allows anybody to submit an offer to me. I also am selling them on SeDo but decided not to park them.
My reasoning is that a lot of people who are searching for their business domain name don't know much about buying domains. So, instead of looking things up in the WHOIS database - they simply type a domain name in the browser window and see "if it's taken".
Most parking services have a "buy this domain link" on their landers. Anyone half saavy about the internet at this point should know what a parked domain page looks like and that parked domains are often for sale.
If you have domains that get traffic, why not park them? Selling and parking are not mutually exclusive and you might make enough money to cover your registration fees.
I agree with cfx211. I would consider parking them with a parking provider who can provide statistics, ad revenue, and most of all the forsale page. Most likely the domains will at least generate enough ad revenue to cover their renewals until you sell them.
Some like Sedo's basic service only provide unique visitors while others like Fabulous and Parked can also provide traffic analysis to show search queries and countries your traffic is coming from. You will want to look around at the various parking providers and evaluate the one who will work best for your situation.