Had the same issue when 7.0 came out. You have to go into eBay and select Site Preferences. Next go to the Payment from Buyers and the last parameter is "Use Sales Tax Table". Set to Yes and you will be taken to a screen showing all the States. For the states in which you charge sales tax, enter the rate and save. GarageSale will now work properly.
After I discovered the problem, GS said they would give an error message when you tried to put a sales tax code in GS for which there was not an eBay entry. Apparently, they have yet to do so.
GS 7.0.5 - 18,000 items - Long time user