An exchange is a market where you buy stuff, namely, other money.

Also, people sell gift cards and packets of cash on robosats all the time, and those are some of the things you traditionally find on other darknet markets.

Also #2, plenty of people on monero DNMs create offers where they basically say "send me X xmr and I will send you Y btc or Z ltc or Q tron" where X, Y, Z, and Q are based on the current exchange rates. So darknet markets are for exchanging currencies too, just like robosats.

Also #3, nothing stops you from selling non-moneys on robosats. You can create your own trading pairs, so if you want to sell bottles of ketchup, just make an offer that you'll give someone 30 bottles of ketchup for .0004 btc or whatever.