As I do this for a living, I will give you a hand.
There are 2 ways to go about it. I would recommend going both ways.
The first thing you do is call snap-on and find out who your local rep is and make a call to him and ask him to come evaluate your tools. Most dealers are stand up guys and will not charge you to do this, but will have to be on their down time, ie after last stop, or a sat or sun.
He will give you a higher value of the tools than you can sell them for. You will know this once he gives you the value and you ask him to cut you the check. Actually they do sell used tools, but usually not whole collections, and most will not deal with older tools.
The second way, is a long and exhausting way, but will get you the most money for your tools. If you can id the set write down the # and go to snap on dot com and you can see what it is new. If you can not id a set, take 1 item out either socket, or wrench, find the stock # IE oex, soex, oexm etc..etc... and put that # in search box on snap on dot com. When it comes up a price will be shown and below that will show in what sets does it come in. You can then see which set you have, and click on that set and see what the new cost is.
Having a complete list of all sets, and the other loose tools will get you the most money for the tools. If you have a yard sale and do not have your list, you will probably not make much and find that the tools are gone. Meaning you will sell the tools, but not for maximum potential.
You make the most money like so,
Sell it in sets, complete as possible on eBay.
You make the least amount selling it all at once. I know this because this is how I often buy tools. I buy estates out, and I buy whole box sets. I then piece it out.
A good rule is if the tools are not all beat to chit, and are in decent eye candy condition, you should get 50-60% of new, while selling on eBay can get you up to 80% of new depending on what it is. Completed listings search is okay, but do not count on fetching that exact price, nor do you cut yourself short either, as sometimes you will get more than that price. It is all based on who needs what and at what time they need it. Good luck and if you need any more help just ask.