I have a common dilemma that I'm sure others here have faced: I have a huge, sealed Lego inventory and in moments of weakness, I get the urge to build a set. Staring for months at my rack of sealed Death Stars, Tumblers, and Sandcrawlers can break down the resistance of even the most hardened investor.
Anyway, once you've made the foolish decision to build a kit, I think there's some things you can do to preserve as much value as possible for later resale. Here's a couple of thoughts and ideas:
1) Face it, you're going to lose some value. This is unavoidable. A sealed kit with unopened bags will always be worth more than a built kit, no matter what else you do. If at all possible, try to buy the same kit already used, or with an open box, or some other variant. Break the seals on your new kits as a last resort. That said, here's some further ideas...
2) Obvious things are keep the box in as good a shape as you can (I store my opened boxes right along with my sealed kits), as well as the instructions. If the kit includes stickers, bite the bullet and don't use them. Keep all stickers on the sheet. For things like UCS nameplates, you can use color xerox copies for display, keeping the stickers intact. And this goes without saying: don't lose any parts or minifigs, including the few bits of extra parts.
2a) When selecting a kit to build, if you have multiple in your collection, build the kit with the worst-shape box. Also, obvious, but I carefully evaluate all my copies, and pick the one with the most damage, perhaps a broken seal, maybe some crease marks or a bit of crush-age.
3) Not so obvious things. I carefully open each sealed bag with a pair of scissors (more on this, below), and I SAVE ALL THE BAGS. Once the novelty has worn off and I decide to break down the kit, I re-use the bags. Here's the tricky part: I also use the instructions to reverse-build the kit, and I carefully place the disassembled parts back into the original, numbered bags. The goal is the next builder opens each, numbered bag and all the right parts (including extras) are present. This may seem excessive, but it certainly can't hurt value, and I think it will help on resale.
3a) For those kits that have multiple bags for each, numbered step, I've often taken pics of the bag contents prior to build. I'll try to re-package parts back into the same bags, but this isn't strictly accurate, and probably overkill.
4) All those tiny parts that come as extras in each bag? When building, I put the extras back into the appropriate numbered bag, then store them with the box. That way, the extra bits will be in the right place when I disassemble.
5) Make sure on disassembly that your kit is as clean as possible. This can be tough if its gotten dusty. I use soft-bristle brushes first to clean, followed by compressed air if needed. As a last resort, I'll run parts in the sink with the sprayer and warm water, followed by towel-drying and the blowdryer to remove excess water.
6) If you cut open bags with scissors along the long, non-sealed edge (the part that's continuous and wraps around), I've found you get a beautiful, clean re-seal of the bag using clear packing tape. I wrap the tape around the cut, then use scissors to trim the tape even with the sealed edge of the bag, top and bottom. Harder to describe than show (and I could get a picture of this if anyone is interested). When done, the bags look fantastic. You're not going to fool anyone into thinking its a sealed set, but it will look like the best, built set you can sell.
7) Much of this effort is for naught if you don't take good pics of the kit, both built, and after disassembly. Put these pics in your auction, if you're selling on eBay. It works wonders, at least for the half-dozen sales I've done in this manner.
8] Maybe all this is overkill and I'm anal. You're probably right. None-the-less, the steps above won't hurt, and there's a bit of extra fun in disassembling kits carefully and getting them back to as close to as-recieved condition as you can.
BTW, the few sales I've had of kits given this treatment have all gone slightly above the usual resale prices on eBay, and 2 of those kits went for almost (within 5%) of new, sealed prices.
Thoughts? What do you do when the urge to build strikes?
Figure 1. End result of foolish Lego investment decisions...
edit:
Thought a few pictures might help illustrate my suggestions. Here's a Volvo 42030 Loader I built, then disassembled. Note the sealed bags. This particular kit didn't have numbered bags, but you'll get the idea. It's very hard to tell this has been opened and resealed, as you can see. I find buyers really do respond to this care, and it doesn't take much effort.