just that they are duped. People around here have been trading duped items for a long time. Essentially you would be buying a fake item on the black market, so it is always a good idea to keep from announcing your new gains to the world. The tend to frown on such things. The more attention you bring to yourself the more likely you are to be frozen.
Pretty much. Don't announce nothing unless the boards already think you're legit and cashed-up. Have cover stories for how you got your neopoints (you're an RSer, that's enough. You saved up so long and so hard for it. Maybe you got it a while ago and were waiting to album/sell it. Etc.). The stamps are pretty much safe to buy and use, you just need to keep above suspicion of other users so they don't report you. Which... is pretty easy. Users don't check each other's stats very much anymore now that user lookups have captchas. And as we've already said, most of the top users are cheaters so you're in good company, helps skew the idea of what's real and what isn't (heck there's a very active neoboards user with a less than 2-year old account with the Virtupets stamp album avatar).