"Do inflationary currencies could cause people to hoard commodities?" Of course they do! Dont you think that during the German hyperinflation people refused to sell of course they did!
No, they refused to exchange for currency. The reason they couldn't exchange was because they had no functioning medium of exchange, not because they had an inflating currency. Unpredictable or absurd inflation or deflation can make a currency unusable. (Which is no problem if there are other currency options but a big problem if there aren't.)
"Regardless of the properties of the currency, inflating or deflating, stable or risky, since they affect you and I equally" What? I cant make heads or tales out of this comment. If the currency you gave me inflates and the value of the car goes up how does that affect us equally? You got a car that appreciated in value and I got your devalued cash.
It affects us equally because it reduces the value of the money I offered to you in exchange for the car just as it reduces the value of the money you received in exchange for the car. This perfectly cancels out.
For example, say I have a piece of gold that will be worth $50 more tomorrow. We both know this, and it affects the way we value the gold equally. To whatever extent it makes me want to hold onto the gold more (to gain that future value) it precisely equally makes you want to get the gold from me (so you can gain that future value). Presumably, we both equally value a $50 gain from today to tomorrow, so it should not discourage the transaction -- you want to buy it from me as much as I want to keep it, so we should agree on a fair price for it.
Or, for inflation, say I have a piece of gold that will be worth $50 less tomorrow. I will be more willing to part with that gold because I don't want to sustain that loss. But you will be precisely equally less willing to accept the gold since you don't want to sustain that loss. This perfectly cancels out.
Or, to view it one last way, say the government was about to impose a $100 one-time tax on cars such that whoever owned the car had to pay a $100 tax on a particular day. (This is precisely analogous to a currency deflating -- the value goes up in the future after the tax is paid.) This affects the buyer and seller of the car equally -- they each value the car $100 less before the tax is paid than after. So it won't discourage trading the car, it will just change the price.
"I should consider the car worth more units of that currency than you do." This assumes that one human being is capable of processing all of the millions of signals that set price. One person can not know the market as a whole knows.
That doesn't make any difference. (Unless you want to argue that unpredictable inflation/deflation creates friction. I agree, it does. But predictable inflation doesn't, nor does predictable deflation.)