First off I don't believe that a field strip will make a gun used. Using that logic racking the slide, pulling the trigger or dry firing it will also make a new gun used used.
On your part you should check out some YouTube videos on how to field strip and clean it before you make the trade. I know it's not hands on experience but it should help you become knowledgeable about the process. Then go discuss it with the salesman. Make sure he/she understands you are a serious buyer. If he/she won't let you do it have them at least describe the process and question them on any issues/problems you might incur. Could also let you know their knowledge vs BS level. They also might not know how but hopefully they would tell you that.
Not familiar with the handgun being discussed but as others have stated there are guns that can be broken down and reassembled in a minute and there are others that take an act of God to get back together. That is a definite factor to consider.
Really, the bottom line is your comfort level. Is this the person/store you trust and want to do business with. If so, go for it. If not, keep looking.
BTW, I learned that gun stores are really playgrounds for many adults. I've seen them do bizarre things with the display models. The chain stores especially rarely say anything so they don't offend the customer and lose any future business. For a new gun I will not take a display model. If they don't have one New In Box I'll have them order one, go somewhere else or just wait. But that's me personally.
As far a used gun I would not even consider a purchase without a look at its internals for integrity (rust, broken/bent parts, idiot with a Dremel tool who polished the feed ramp to death, ect). You can bet money on the fact that that they are going to take your trade-in apart to check it out before they commit to taking it so shouldn't you have the same opportunity if you are buying used. If not, walk away.