Test the zippers to make sure they move smoothly and easily. Feel the fabric. Make sure you'd like to sleep against the liner fabric. Check the bag for overall workmanship, fit, match, etc. What it's made of and how much is used can greatly affect the temperature rating of the bag under various conditions.
Synthetic fills are basically synthetic fibers bonded into sheets.
They differ in how fibers are made - some are long and continuous, some are short, most are hollow, some have many hollow sections or tunnels running through the length of the fiber. Other synthetics differ in how they're bonded together, some in a fluffy, air-trapping matrix. But they're all designed to hold and trap air. The more efficiently insulation can trap air, the warmer that insulation will be per pound of weight. It is a comfortable, cost effective synthetic fill.