Because the verb is conjugated as a second-person plural, and "merchants" alone would be a third-person plural.
Thanks for answering. Would the sentence "You all who are merchants estimate the prices" mean same as this sentence" ?
That would make two separate sentences: "you/you all estimate the price" (principal clause) and "who are merchants" (relative subordinate clause). It would become something like: "Vos, qui mercatores estis, pretia aestimatis". The concept of course it's the same, but the construction is clearly different, both in Latin and in English. You can read more here: https://www.thoughtco.com/relative-clauses-in-latin-117781
(I hope I understood it correctly, maybe you meant to ask how you would translate it if you meant: "those of you who are merchants"...)
"Vos mercatores" not just "mercatores".
And both in the same case (nominative) too, so they refer to the same things/people. Well "mercatores" can be accusative too, but that would be the same as "pretia" and that doesn't make much sense (prices-merchants / merchants-prices). Remember it can't mean "merchants' prices", that would require the genitive ("mercatorum pretia").
Actually thinking about it a bit more, "pretia" can be nominative too. So "You prices estimate the merchants" also works, at least grammatically.
Thanks for answering. Are pretia and mercatores same as nominative when used as accusative ?