"They sell a lot of spices and oils."
Translation:Ili vendas multe da spicoj kaj oleoj.
Its similar to the difference between 'Mi trinkas tason de akvo' and 'mi drinkas malvarman akvon'. In the latter cases the objects of the verbs are 'a lot' and 'a cup' respectively, and what its a lot of or what its a cup of is seperated from the object enough (by a preposition) that its not in the accusative. Its a difficult and counter-intuitive distinction becauae to your ears 'a glass of water' or 'a lot of spices' sounds like one object.
In the former cases 'multajn' and 'malvarman' are adjectives that still form part of the object noun, so both will be in the accusative.