rara

Say malofta unless you mean the collector's sense. This is the honest headline. The dictionary carries rara in two senses - prized because it is scarce, and simply seldom found - and then prints a remark telling you to prefer malofta for the second and altvalora for the first. That remark is not a nicety. Malofta is built from ofta (frequent) and the mal- prefix every learner has on day one, it is instantly transparent to a speaker of any first language, and it is what most Esperantists actually write. Rara is a borrowing that duplicates a word the language could already build, which is the exact thing the affix system exists to avoid.

So when is rara right? When the scarcity is the point and the value comes with it: a rare stamp, a rare edition, a rare wine. Raraĵo - a rarity, a collector's item - is the derivation that earns its place, and it is commoner in print than the bare adjective. Rare as an adverb means seldom, and here malofte is again the safer choice.

Not a false friend, but shaded. English rare has a third sense - underdone meat - which Esperanto does not give rara at all; that is duonkruda or sangeca. And note that rare meaning strange or remarkable, as in a rare talent, tips towards speciala or stranga rather than rara. The Latin behind the word is thinness, not oddity: rara air is thin air, and the scarcity sense grew out of things being thinly spread on the ground.