Sources for used guns include GunBroker.com, pawn shops, and all gun shops.
The best source for a used gun would be a reputable gun shop.
In my own experience, a well-maintained used gun gives the buyer the best value available for his money. Jean and I own very few guns that were purchased new.

I suggest establishing a friendly, personal relationship with a good local gun shop, and then doing the same with a good local gunsmith too.
Once a friendly relationship has been established with a shop, you could arrange to buy a used gun with return-for-credit privileges. The next step is to take it to the gunsmith for evaluation.
If the gunsmith advises against the purchase, take the gun back to the store and trade it for another possibility. And so on.

I purchased both new and used guns from individuals, gun shops and online. There is a difference between a gun being used and those that have been abused.

It is great to be able to handle and inspect the weapon prior to purchase to make sure it is in good working order. Buying used guns online can be tricky sometimes. Ask questions of the seller and try to get definitive answers. Keep in mind though if the seller is not the original owner he may not know the history of the gun, round count and so on.
Research the gun you are looking for or at on different sights or shops and compare prices. Antique or "rare"guns have very subtle differences that can greatly effect prices. I have not gotten a lemon yet, knock wood.

Since you state you are new at this then Steve's advice to find a local gun store would be good to take and leave the online buying for a later point down the road. Sig Sauer has a line of used factory refurbished pistols that carry a one year warranty you might also look for.....JJ

You can find a lot of very good used pistols that will be worry free and usually less expensive than new if you shop smart, I like Steve's advice about getting to know and be known at your local gun shop(as he said a reputable shop that will stand behind what it sells new and used). You may wish to also do an online search on 'what to look for when buying a used pistol' there are several good write ups that may help get you started in learning how and what to look for to find a good used pistol.

There is nothing wrong with buying used guns. If you are unsure of the condition of the firearm, take it to a gunsmith for evaluation. I always look for used guns first before resorting to buying a new one. If you buy from a private party, protect yourself and always get a valid bill of sale that can be traced, i.e., with a drivers license number or similar government based id. If you can't get the proof as such, do not buy it regardless of the price.

i have bought 2 used guns. One G19 for my brother, and a G30sf. They both worked fine. The G19 recoil guide rod broke but that is an easy fix. If its your 1st gun I recommend new just for peace of mind, but my brothers first gun was used and its still 100%.

This article by Mr. Grant Cunningham is a pretty good guide to selecting a used handgun. Obviously there are criteria that some of us would include, or exclude. Please adjust to suit, keeping in mind that I am not the author.

Because it's a long article, I'll post a link to the document that resides on my FTP server. I have no idea where the original article was posted, or when.

Buying a used gun

Very good advice here.Since you're a "newbie",have somebody versed in the type of gun you want to tag along or do the gunsmith gig.If the seller won't let a locally reputable smith look at it,forget it.Having a friend go along usually ends up well but there are hidden things that aren't always apparent on a cursory inspection.I've been bitten by it,but I didn't detail strip an auto so it's my fault.There are basics to look for that differ between autos and revolvers that are easily tested,but a thorough look is the only true way.

Having said that,the overwelming majority of the time if the gun appears in good shape,taken care of and function checks are fine,it's a good bet to say it is fine.I've bought alot of used guns and only the one was so called bad.My intention on the purchase was to rebuild it into a match gun so it wasn't a huge issue,but it was an issue that was a possibility with that brand,and correctable if you knew how.

Hello to All Again,
And thanks for all the great advice.