There are a few things to consider when shopping for a used fireplace. Keep in mind that if the fireplace you've purchased is not deemed safe by a licensed gas technician, he or she can not legally install it.
A few things to consider and check for....
How old is the fireplace? and will it run as efficiently as newer models? If not, your higher gas bills could quickly eat up any savings you've acquired, by buying used.
Does it need replacement parts? The cost of replacement parts can quickly add up, and possibly end up costing you the price of a new fireplace - that would include some warranty!
What is the model number? This is important. Every fireplace should come complete with a rating plate attached to it somehow. It will either be chained to the unit, or stuck on as a sticker. If this plate has been tampered with, removed or painted over, it's time to walk away. If the rating plate is gone, there is no way of knowing whether the fireplace has been recalled, how to get replacement parts if needed, or how many BTU's it puts out. If it is deemed unsafe for use, it will be red tagged ie..shut down by a gas technician.
If you happen to come across a suitable unit, that meets all of the above criteria, consider contacting a local and reputable gas technician to assist you in the purchasing decision. A gas tech with inspect the unit with a fine tooth comb. He/she will not only be able to tell you whether the unit needs any refurbishing/new parts, but also whether it is the correct application for your particular home and needs.
Best of luck with your search!!