Looks like all is well now. My first user was having problems posting...but I was able to post a test ad...and several others have, since, posted ads with no problems.
I set the system up for "free ads", but a charge for the "featured"...but, when a few got to the "featured ad" option, they were unclear whether the basic as was free or not...so I removed the "featured" option. I may have to see about adding a line to ensure users know the featured ad is NOT required.
I'm now inserting a few "placeholder" advertisements using wp125, which was recommended here (thanks!). I had always considered using 250x250's...but the 125's seem to be the "norm" for blogs. "More ads" sounds good to me! ;o)