The EOS 6D has a shutter rating of 100,000 actuations. This means that Canon expects half of the 6Ds sold to have shutters that last longer than 100,000 cycles and the other half to last less than 100,000 cycles. As with any MTBF estimate, there is a Bell curve involved and most samples will land fairly close on either side of the center while a few outliers will last much shorter or much longer.
14,000 actuations is 14,000 actuations. It really doesn't matter if they've been accumulated over several years or only over several months. Unlike other things, such as vehicles, cameras aren't usually parked out under the UV rays of the sun and in the weather when they're not being used. They don't have lubricants that need to be changed regularly due to chemical changes that take place whether the vehicle is driven or not.
In the case of the 6D in question, the shutter has been used approximately one-seventh of its expected life. If the price you are paying is less than six-sevenths of the price for a new 6D in your area it is probably a good deal if the camera checks out in all the other ways that you should look at any used piece of photographic gear. The fact that the seller is giving you a better warranty than Canon gives new (at least here in the U.S. all new Canon gear comes with a 1 year warranty) removes a lot of the risk of buying used gear.