at the expense of whom listens to them -> at the expense of those who listen to them

I'm afraid Reverso disagrees with you and agrees with me...
http://context.reverso.net/trad...the+flatterers

English is so diverse
https://tatoeba.org/fra/sentences/show/2024265

I'm pretty sure that Reverso simply took your sentence. It's not the first time that I've seen sentences written on Tatoeba show up there.

It did take your sentence. If you move your mouse under the right-hand part of the pair and look at the rightmost icon under the sentence, you'll see that it lists Tatoeba as the source.

In Tobias Smollett : "Peregrine is celebrated as a Wit and Patron, and proceeds to entertain himself at the Expense of whom it did concern."

How is this different ?

I have a theory. The difference is that in that example “whom” is really object case while in your sentence it is not: the preposition “of” precedes it, but it’s the subject of the relative clause, so it should take the form “who”. ☺

+1 Lisa

+1 from me, too.

To elaborate: English pronouns that decline (unlike "you", "it", "which") have a subjective case ("I", "he", "she", "we", "they", "who") and an objective case ("me", "him", "her", "us", "them", "whom"). The objective case serves multiple purposes: it can indicate a direct object ("John saw her") or an indirect object ("John gave her a present"), it can show that the pronoun is governed by a preposition ("speak well of her"), and it can indicate emphasis, especially when the pronoun stands alone or at the end of a sentence ("Who's there? "Me!"). In some situations where people generally use the objective in casual speech ("taller than me", "it's me"), there are rules that suggest that the subjective case should be used instead ("taller than I [am]", "It is I"), especially in formal speech. Such rules tend toward the prescriptive end of the spectrum and are often favored by the same people who favor rules like "Do not end a sentence with a preposition" and "Do not split an infinitive", often based on questionable reasoning (for instance, analogy with Latin). Nonetheless, the rules have a tradition, and speakers are often best off rewording sentences to avoid violating them.

As Lisa suggests, "Peregrine... proceeds to entertain himself at the Expense of whom it did concern" is acceptable (or at least was in the 18th century) whereas "Peregrine... proceeds to entertain himself at the Expense of who flatter him" sounds strange because it puts the preposition "of" before the subjective "who".

The natural way that an English speaker would resolve this now (and perhaps then as well) would be to use "of those who" or "of those whom". This lets the word "those" be governed by the preposition "of" while the rest of the clause sentence is free to use "who" or "whom" according to whether it is followed by a subject or an object. In this case, we have a subject, "those who listen", so "at the expense of those who listen to them" is better.