The thing to keep in mind is that part of the reason why Lego is valuable is that it's difficult to keep all the pieces together, especially when kids play with them. It's a challenge and here's what I've come up with...
- Once a set is done being played with, we bag up all the pieces, instructions, comic, etc and put it back in the box. If there's any missing pieces, I'll write it down on a sticky note and put it in the bag. I also have a note on my phone that has a list of missing pieces for sets, that way if I find a piece I can reference the note.
- Buy some sets that are really fun, but don't have collectors value. Early this year, the Galaxy Squad stuff was on such clearance at TRU it was almost like getting them for free. And surprisingly, they were some excellent builds. Tons of fun, and if we lose some pieces, no big deal. I'm hoping next year's January super clearance will be the Ultra Agents sets.
- I tell my son that there are some sets for looking at, others for playing with. The Ultra Agents Ocean HQ is for playing with, the Sea Cow is for looking at. Some sets are just better for playing with, and hold up better for play. As awesome as the Sea Cow is, it's hard to move without some part coming off. On the other hand, other than the ladders in the back, the Ocean HQ holds up better for play. But, the Gatling dot guns are a parent's nightmare. Some of the Power Miners sets are surprisingly resilient too.
- For many play sets like the Super Heroes series, much of the value is in the minifigs. I don't have any guilt about stashing them away, and out of his and his friend's hands. I have a bin full of minifig pieces that they could build any number of characters that function, but not built to spec. They can play with those, not Alfred or the Chrome Stormtrooper.
- I strongly suggest sorting by shape, not by color. It's much easier to spot red in a sea of 2x4s, than it is to find a 2x4 in a sea of red. Color doesn't affect form or function as much as shape does, especially if the brick you replace it with is hidden. Also, if you sort solely by color, the small pieces will fall to the bottom and will be difficult to find. Those are the things that will kill you on a rebuild. Once you get the sorting done, the rebuilding is so much easier and takes less time. My elderly, retired mother helped me sort a ton. I told her that it would help her to ward of Alzheimer's. I told her she should suggest it to her friends by telling them how much sharper she is from sorting Lego, but she just rolls her eyes.
- Sometimes it's good to order spare parts when they are cheap. For example, sails. I bought a spare set of sails for Destiny's bounty for a little over a buck. Well worth it to save the value of a set that might be worth alot.