Most online dictionaries are put together by teams of anonymous volunteers, and you may notice patchiness. Only a few English-Esperanto dictionaries have been published; the most commonly encountered, Wells's, is a pocket-dictionary, and for some words only the most basic translation is given.
The English word scoff is intransitive, but otherwise it is almost a synonym of mock in the sense of expressing laughing contempt. Scoff doesn't specify anything about noises or the nature of the ridicule. However, whatever concept you have in mind, it should be fairly straightforward to find a way to express it.
Esperanto is a constructed language in more ways than one. The basic idea is that, if you are stuck, you figure out the essence of what you mean, and then construct a word that captures it using the components you know. The last major component in the word is the governing idea. If it is hard to cram it all into one word without making it ambiguous, put the rest in an adverb, or just spell the whole idea out.
Here are some components that may be useful for your example: ridi (laugh), moki (mock), imiti (imitate), snufi (sniff), tusi (cough), rikani (sneer), ronki (snort, snore), sin deturni (turn away), eviti (avoid).
For example, while rikani appears in the dictionary under sneer, that concept might also be expressed as subridaĉi (to laugh unpleasantly beneath [the surface]). Similarly, moki itself could be expressed as ridindigi (to make worthy of laughter).
So you could consider ridtusi (laughingly cough), tusridi (coughingly laugh) snufmoki (sniff-mock), ridetaĉi sin deturnante (snicker turning away), mokeviti (mockingly avoid), and so on. Constructing expressive words takes some practice, so checking the dictionary first is always a good idea, as it will show examples of others' solutions. (Remember that these are not really "coinages" or "neologisms" any more than gooseberry-flavoured is a neologism in English simply because you have never seen it before.)