Just because one buyer was willing to pay $X doesn't necessarily mean other buyers will as well. Maybe that first buyer isn't interested in buying any more since he already has what he wants.
If you don't mind waiting, potentially quite a long time, then you can feel free to leave your selling price at $X, or even increase it. But if you need the product sold soon (for example, so you have the money to spend on your own expenses) or if it's a one-off sale and you just want the thing out of your house, then you might want to consider lowering the price to make it sell faster.
There's also the question of other sellers. Maybe you're sticking to your "poven" price of $X, but if someone else starts selling the same thing cheaper, buyers will most probably go to him instead of you. Again, it's a waiting game. If you're content to wait until all other sellers have run out of supply to sell, you can maybe still command your desired price. But if you can't wait that long, or if other sellers are not likely to run out of supply any time soon, you might want to consider lowering your price to compete.
And then there are other market factors to consider, aside from basic availability of supply. Changes in public mindset can influence how much people are willing to pay for a given product. A product that's popular due to being a fad or having some association with a popular political mindset might be able to command high prices, but over time those interests may fade from the public opinion and the buyers of the product will correspondingly dry up.
Finally, there's the the question of perishable goods, or "newness" in the case of things like video games. This may not apply so well to Bitcoin, but some products are more valueable when they're new/fresh, and will natutally degrade in value over time. So you need to set your price accordingly, and you may need to lower it if it doesn't sell quickly enough, because it's simply less valueable after time has passed.