Honestas mercatori deest, si pretium iniustum petit.

Because deest works with the dative of the person affected.

deesse literally means to be lacking to someone. So:

• honestas mercatori deest = honesty is lacking to the merchant
• more natural English: the merchant lacks honesty or honesty is lacking in the merchant

So mercatori is not the subject. It is the person to whom something is lacking.

Deest is the 3rd person singular present active indicative of deesse, which is a compound of:

• de-
• esse = to be

Its basic meaning is to be missing, to be absent, or to be lacking.

In this sentence:

• honestas ... deest = honesty is lacking

Since the subject is singular (honestas), the verb is singular: deest.

A learner may also notice that compounds of sum are very common in Latin, and many of them have their own case patterns. deesse commonly takes a dative.

The subject is honestas.

That can feel backwards to an English speaker, because English would usually say:

• the merchant lacks honesty

But Latin says:

• honesty is lacking to the merchant

So grammatically:

• honestas = nominative singular, subject
• mercatori = dative singular, the person affected
• deest = is lacking

Honestas is a noun meaning something like:

• honesty
• integrity
• uprightness
• moral respectability

In this sentence, honestas is not just about telling the truth in a narrow sense. It suggests moral uprightness or fair dealing. So the idea is that if a merchant asks an unfair price, he lacks honestas.

Because it is the direct object of petit.

• pretium = price
• iniustum = unjust, unfair

Together they form the phrase pretium iniustum = an unjust price

Since petit means seeks, asks for, or demands, it takes a direct object:

• petit pretium iniustum = he asks for an unjust price

Both words are accusative singular neuter:

• pretium = accusative singular neuter
• iniustum = accusative singular neuter, agreeing with pretium

Here petit means asks for or demands.

The verb petere has a wide range of meanings in Latin, including:

• to seek
• to aim at
• to ask for
• to request
• sometimes to attack

In this business context, with pretium as its object, the meaning is clearly:

• to ask for a price
• more naturally, to ask an unfair price or to charge an unfair price

So although petere can mean different things in other contexts, here it is about demanding a price.

Because Latin often uses the present indicative in general statements.

The sentence expresses a general truth or principle:

• If a merchant asks an unjust price, honesty is lacking to him.

That is not about one specific event in the past or future; it is a general rule. So Latin naturally uses the present tense:

• deest = is lacking
• petit = asks for

This is very similar to English: If he asks an unfair price, he lacks honesty.

Si means if and introduces a conditional clause.

So the structure is:

• main clause: Honestas mercatori deest
• conditional clause: si pretium iniustum petit

Literally:

• Honesty is lacking to the merchant, if he asks an unjust price.

More natural English:

• A merchant lacks honesty if he asks an unjust price.

Because this is a straightforward, real condition, Latin uses si with the indicative mood.

Yes, the understood subject of petit is the merchant.

Latin often leaves out subject pronouns when the verb ending already shows the person and number. Petit is 3rd person singular, so the subject is he/she/it from context.

Here the context makes it clear that the subject is the merchant:

• mercatori in the main clause refers to the merchant
• petit means he asks for
• so the sense is: if the merchant asks an unjust price

Even though mercatori is dative in the first clause, it is still the person being discussed, and Latin can continue with an implied subject in the next clause.

Latin word order is more flexible because the case endings show how words function.

English depends heavily on word order:

• The merchant lacks honesty if he asks an unjust price.

Latin can arrange the words more freely:

• Honestas mercatori deest, si pretium iniustum petit.

This order highlights honestas first, which gives prominence to the moral quality being discussed.

A more English-like arrangement would still be understandable in Latin, but the given order is natural and stylistically good.

Classical Latin does not have definite and indefinite articles like English the and a/an.

So:

• mercatori can mean to the merchant or to a merchant
• pretium iniustum can mean an unjust price or the unjust price, depending on context

In this sentence, English normally uses the indefinite sense:

• a merchant
• an unjust price

But Latin does not need separate words for that.

Iniustum is the adjective iniustus, -a, -um in the accusative singular neuter form.

It agrees with pretium, which is a neuter noun.

So:

• nominative singular: pretium iniustum
• accusative singular: pretium iniustum

Because many neuter nouns and adjectives have the same form in the nominative and accusative singular, the phrase looks the same in both cases. Here, however, its function is accusative because it is the object of petit.

Yes. The Latin structure is not identical to normal English structure, so several good translations are possible. For example:

• A merchant lacks honesty if he asks an unjust price.
• Honesty is lacking in a merchant if he demands an unfair price.
• If a merchant charges an unfair price, he has no integrity.

All of these reflect the same Latin grammar and idea. The exact English wording can vary, especially for honestas and petit.