Years ago, I went to a barber and got a haircut that took no more than five minutes. I go with simple haircuts, and he had basically run some clippers over my head and used scissors to blend what was left. At first, I was a bit taken aback, and thought that perhaps I should tip less than usual (and indeed wondered whether I should be charged less than usual altogether), but very quickly realized that this was perverse; the haircut I had received was not, in the context of my preferences, inferior in any way to other haircuts I have received, and I'm better off having the other (say) 15 minutes of my time to (say) squander writing blog posts on the internet. Ceteris paribus, we both benefit from his having finished more quickly; I left my usual tip, leaving the pecuniary terms of trade unchanged from those in which we both lose more time.
Liquidity, like speed, is a benefit to both the buyer and the seller; both are a bit hard to analyze with supply and demand for this reason. (My go-to deep neoclassical model, from Arrow-Debreu, treats a quick haircut as a different service from a slow haircut, and as such treats them as different markets, but they are such close substitutes that it's obviously useful to treat them as in some sense "almost" the same market.) There may well be other ways in which different instances of a good or service differ in ways such that the quality that is better for the buyer is naturally better for the seller as well. My interest especially is in market liquidity, and I wonder whether distilling out this aspect provides useful models for some of the important phenomenology around that.