Who are "Yoshevei ha'Machtesh"?
Rashi from Targum Yonasan: Those who dwell in Nachal Kidron, which is deep like a Machteshes (mortar).
Rashi from Pesikta Rabasi 8: It is Tiverya, which is deeper than all of Eretz Yisrael.
Radak: This is a deep place in Yerushalayim.
Malbim: These are Ashirim and merchants who did not put to their heart the breakage of the officers. Most of them went far for their business, like the custom of Am Kena'an, people of Tzur, who were all merchants. They dwelled in a part of the city called Machtesh. Also the residents of Machtesh will wail, for "Nidmah Kol Am Kena'an" (all the merchants were cut off), since Netilei (those who carry) Kesef ceased, and also all business ceased.
What is the meaning of "Ki Nidmah Kol Am Kena'an"?
Rashi: Was broken the entire nation whose deeds resemble the deeds of the nation of Eretz Kena'an.
Radak: Kena'an is a merchant, like "Kena'an b'Yado Mozenei Mirmah" (Hoshe'a 12:8), "Kena'aneha Nichbadei Aretz" (Yeshayah 23:8). Nidmah is cut off, like "Nidmo Nidmah Melech Yisrael", "Nidmu Ami" (Hoshe'a 10:15, 4:6).
Who are "Kol Netilei Kasef"?
Rashi, from Targum Yonasan: All the Ashirim laden with a load of silver and gold. Netilei is an expression of a load, like "Koved Even u'Netel ha'Chol" (Mishlei 27:3).
Radak: They are people who carry Kesef, i.e. merchants - "Over la'Socher" (Bereishis 23:16).