Why doesnt "He sells coffee glasses" work here?
Eh... coffee glasses would be taken more-so to mean "glasses used for coffee" rather than "glasses filled with coffee".
So how would someone say "He sells coffee glasses" instead?
You could use the same word or add in a word to remove the ambiguity like chứa (to hold/to contain). Ly/cốc (chứa) cà phê, or something like Ly/cốc (dùng để) uống cà phê meaning "glass used to drink coffee".
Who says "glasses of coffee" instead of just coffee?