Here is my two cents worth on Switch Barrels.
I have 4 rigs with switch barrels. Three of the rifles have not had their barrels changed in excess of 10 years. The barrels lay in a rack in my shop collecting dust (actually they are in tubes to protect them). The only one that I take the time to swap out is an XP-100 pistol that has three barrels, 6BR, 6Dasher, and 308 Win.
It is easy to do when they are set up correctly, but every time they must be sighted in again. What a pain. But with anything other than Savage Barrel nut style action, Barrel vise is a must.
If you have never swapped a barrel, you need to find someone that has the tools to show you. That way you would have a better understanding of process. Its easy, but it is not like screwing a loading die in a press to size 38 specials.
Regarding buying used barrels, if the barrel was removed from a rifle, there was prob a really good reason, IT DIDN'T SHOOT ANYMORE! Barrels are like tires, when they wear out, throw them away. They make nice tent stakes.
The exception is if you get hooked up with someone that shoots BR or F Class competitively, allot of those guys pull barrels at the first sign of shooting flyers in a match. Those barrels will make very nice varmint rifles, but you are limited on twist options.
JMHO