If everything happens IC and my character has no way to know that what she is being told is bogus but trusts the person saying it (despite the fact that I may know OOCly that such a thing does not exist or is not possible), I would go along.
For example, if someone my character trusts offered a <made up name> Glyph and explained all its properties and their need to sell it, my char would show interest. I, as a player, know such an item doesn't exist but not my character because she has limited knowledge of magic. I can't be sure if the RP is heading towards that (wouldn't be the first RP glyph to appear) but that's part of the fun. Thus, she may fall for the con. That wouldn't happen with a weapon probably, since she has more knowledge in that field than magic.
The fact that you may know OOC that something isn't implemented yet or simply won't, doesn't mean that it can be presented ICly somehow, and it doesn't have to be a scripted RP at all.