I tried the extra sticker / serial number idea, but found the stickers I bought could easily be removed. One of my bigger worries right now is selling current sets that are pricey because they are TOOS, or "unknown" if they're coming back (like 10188 or 75920). If I sell a JW I-Rex for $220, I don't want the buyer finding it at Walmart two weeks later at $99, buying that one and shipping it back to me for a refund. This photo shows one "trick" I've been trying lately for some of my high dollar sales. The picture includes the actual set and the actual packing slip for the sale. If the buyer were to return a different set, I should be able to tell. Selling only a few items per month like this, it's not a big deal to do. Of course, getting Amazon to take my side in a dispute like this could be another challenge. But it helps me sleep at night.