I wouldn't lie to anyone like what you describe, no. I'll offer a price and stick to it or haggle, depending on the item. I have no problems with the concept of "buy low/sell high," and I'm not above seizing the opportunity if I see someone offering a price that I know is a great deal.
But intentionally trying to mislead someone about the rarity/value of something is an incredibly shitty thing to do in my opinion. I'd have trouble doing that for real money. I'm certainly not going to do that for some pixels in a video game.
EDIT - I'd have thought this was common knowledge, but perhaps it's worth mentioning here. If you need more incentive to not lie like what's described in the OP, consider this. There's a good chance that the people you try to convince to overpay with the BS story about how rare your item is will know it's BS. From that point on, they know you to be the sort who
- finds pixels in a video game more important than dealing straight with folks, and/or
- may not really have much clue about gear in this game.
Congratulations, you've just made it onto someone's do-not-trade list or made yourself the future target of more convincing, better educated BS down the road.
Not worth it!