I *think* it actually says that you've asked the question on a holiday Monday when lots of people are outside and playing.
While I haven't sold any CDs in over a year, I have used secondspin and abundatrade many times, successfully and happily. You need to read through ALL of the FAQs for whatever site you're going to use, and KNOW what you have to do. Check out the cases. Check the CDs for CD rot (I think you'll want to google that, unless one or both of the sites has included what it is and how to find it on their pages).
Enter everything in (give yourself lots of time) to whatever site you want to use. When I am in selling mode I put the info into both and work out what is the best for me. They are often a bit different than each other, and then you want to look at their policies to see what works out best overall.
I still have quite a few CDs to sell, and I will happily use either one in the future. I just hit a stopping point because we were scavenging cases from CDs we weren't ready to sell yet, and now we've reached the point where we have to buy new cases or take the hit for the sites replacing them...