I thought 'multe' is an adverb and 'multo' is a corresponding noun.
In this sentence,
Ili vendas multe da spicoj kaj oleoj
it looks quite weird to me why an adverb multe is used instead of a noun 'multon'.
Esperanto Language Stack Exchange is a question and answer site for teachers and students of the Esperanto language. It only takes a minute to sign up.Sign up to join this community
The basic word is the adjective multa for which PIV gives three definitions (my translations; all examples from Zamenhof via PIV):
From the definition 2 follows that you can say Ili vendas multajn spicojn kaj oleojn.
The adverb multe also has three definitions:
You connect a quantifying adverb to a noun using the preposition da, and thus you can say Ili vendas multe da spicoj kaj oleo.
There is also the noun multo which has only one definition: a large group or quantity: mi neniam prenas kun mi multon da pakaĵo.
From the definitions follow that the adjective describes the quantify, the adverb the action and the noun stands for the quantity itself.
The first one speaks about large selection while the other two of big sales volumes. When it comes to the last two cases, multe and multo are interchangeable, but for some reason the latter is very rare. I have seen it mostly without a noun (e.g. Ne restas multo por fari.).