I ran into this exact same issue. The question "Enter the total proceeds for items sold at a loss" seems to imply that if you enter a number here (which you HAVE to), it assumes you will not enter the itemized entries for which you incurred a loss in the itemization under investments later. If you enter a non-zero amount here, AND also itemize sales for which you incurred a loss, it will raise the 'Check this entry' error because it will deduct the proceeds from losses twice.
And the worst thing is, once you are at the 'check this entry' stage, you are left with no option (assuming turbotax online) other than to delete the whole ebay entry, and re-enter EVERYTHING again. You cannot make corrections without doing this other than, I guess, removing the itemized entries for sales that incurred a loss.
At least this is my conclusion of how it works. This could do with some rework / clarification, it frustrated the ** out of me.
