But what would happen if someone started selling using a fixed Bitcoin price? For example, let's say that I want to sell my computer at BTC20, and always at BTC20 regardless of the exchange rate. Would it be a good or bad idea? What consequences would this have?
I personally think that if more people sold with fixed Bitcoin price, the currency would be more stable; but then I'm no expert.
Probably an auction is a better idea to sell something in BTC without referencing yourself explicitly to any fiat price.
Then you will have a price discovery of exchanging this specific computer against BTC, without the forementioned pitfalls of pricing yourself out of the market. Of course, amongst other (dis)advantages, you won't have a fixed price when auctioning items.