In short, the procedure is as follows:
Add items to the product
Check if the items contain any errors. If not then you're done; save the product and start selling it! Otherwise, continue.
Rez the original items from your inventory
Search for the prims that contain errors. The Debug Version can help you with this step; it's explained below.
Fix the errors. E.g.: set a new texture on the prim.
Take the items back into your inventory
Delete the previous items from the product, and upload the fixed items
Repeat until all the errors have been fixed
See the blog post Tools For Finding and Fixing Problems in Products for a walkthrough of this procedure, with screenshots.
Next you need to rez the items and find the prims that contain the problems. If the product doesn't contain many prims then this may be easy, so go ahead and do it. But if the product has many prims then you wouldn't want to start clicking on each prim in turn looking for the one prim that has errors. In that case, use the Debug Version to help you. Click on Deliver debug version: this will add to your inventory a special Debug Version of your product. This version has two differences compared to the real product:
Prims that contain errors are highlighted in bright red
All the prims are numbered, e.g.: "Primitive (Prim #11)". This helps distinguish between different prims with errors (since they all look red).
Fix the Original Items
When you fix your product, make sure you fix the original items, and not the debug version. Typically you would rez the original items and the debug version side-by-side, and then use the debug version as a guide: find a red prim in the debug version, and then look for the corresponding prim in the original items. That's the prim that needs to be fixed.
We don't allow adding these types of items to products in Kitely Market:
Test - items that were created when a merchant clicked "Test delivery" or "Test demo delivery"
Debug - items that were created when a merchant clicked "Deliver debug version"
Demo - items that were created when a user clicked "Try demo" in the Shopping Cart
If you have such an item in your inventory then presumably you also have the original item, since the "bad" item was copied from the original item. Remove the bad item from the product; find the original item in your inventory; and add the original item to the product.