[yk]

There can not really be competition between currencies, since the major advantage of a currency is that everyone uses it. ( Compare the situation to a situation where some people offer meat for arrowheads and other berries for bone knives. )

And besides, sellers will want to use deflationary currencies, because then they raise prices without changing the nominal price. Therefore no one would be able to use a inflationary currency.

[jerguismi]

> There can not really be competition between currencies, since the major advantage of a currency is that everyone uses it.

There already exists global competition between fiat currencies... The competition exists but the competition has bery high barrier to entry.

> And besides, sellers will want to use deflationary currencies, because then they raise prices without changing the nominal price. Therefore no one would be able to use a inflationary currency.

Nope, seller doesn't care if the conversion process is painless. Almost all sellers on the internet accept any currencies - for example, US sellers accept euros without knowing that they accept euros. Payment processors do the conversion.

In the end, the thing that matters is what kind of currency each participant wants to hold. If the buyer wants to hold currency A and seller currency B, they both can still do trade, if there exist somewhat efficient market between currency A and B. And there probably exists, currency markets are probably the first thing to evolve for any kind of usable currency.