Hi Qster,
Yes, for the most part, an auto-relist profile like you have described will behave in the manner that you expect. When an item is set to relist automatically with no changes, the relisted version of the listing will be effectively the same as the old listing was when it closed. Therefore, if 3 of the 8 items you had available sold the first time around, the item would be relisted with a quantity of 5.
Also, the "Lower The Start Price by" option will work for fixed price items as well so, if you use this option to relist your fixed prices items automatically, the fixed price of the relisted version of the item will be reduced by the percentage or dollar amount you indicated in the auto-relist profile.
However, if you had 10 widgets in your inventory and you posted a fixed price listing with a quantity of 8, the auto-relist profile would not be "smart enough" to post the remaining quantity that is available in your Auctiva inventory when the item is relisted. It would just relist with a quantity equal to 8 minus the number of widgets that were sold from that fixed price listing the first time around.
If you have any additional questions or concerns, please feel free to contact our customer support team using the web form on the following page of our site: http://www.auctiva.com/help/requesthelp.aspx
-Mike