If I buy the kit brand new I generally just make sure all of the sprues are present. I also open everything right away to get the decals into a ziploc bag (article in FSM over the last year or so about moisture and bugs actually destroying decals over time).
If I get the kit off ebay or something, I always try to ask beforehand if all the sprues are present. Most sellers are pretty cool, some simply won't respond, and some frankly don't know. When I receive the kit, I always go part by part through with the instructions if the box had been previously opened.
I recently tracked down a 1/32 F/A-18 and bought the thing and found it had several pieces that were not accounted for (fortunately it was a couple pieces of ordnance, which the kit is already heavy in, and the pilot figures so ultimately it was possible to replace it with aftermarket stuff). I also purchased an aircraft ordnance set that was missing the decals and several missiles. The seller had gotten the thing at a flea market and simply didn't know, and it would have cost more to ship back that I paid for it, and I can work around it. Finally, I recently bought a 1/350 Tamiya Missouri and upon receiving it I opened the thing (still in original shrink wrap) to find the hull was actually cracked. It's a clean crack and I think I can work around it, but it certainly made me take a much closer look at the other parts to make sure there wasn't something I couldn't work around.
So anything you get I highly recommend just checking out and storing the decals. Of note though, I fully intend to build every model in my stash (don't we all!) so I am not looking to resell any. But that's my opinion of course.
Groot