When I try to post it says "The domain, www.donesmart.com does not appear to be a valid domain (make sure it's just the domain, not a URL or website address)."
Thats strange. To find out the problem, I tried to list your domain in Marketplace (both as domain and website for sell) and it works as expected. It means I could list it easily without any problem. I just need to add a DNS (TXT) record or upload HTML file (into server). After Verification completed (to prove domain ownership), the domain/website could be listed easily in marketplace. If you get errors, you should enter only the domain (DoneSmart.com). If it still failed, use another browser because it could be browser specific problem. Deactivating any extension or addon could help too. If it is possible, upload screenshots while the problem happened.
We know the error you get when you *don't* post the domain, so what is the error you get when you post the domain?