What is this gift to the king?
Rashi: "Ani Sarshish la'Melech ba'Yam Im Ani Chiram Achas l'Shalosh Shanim Tavo Ani Sarshish..." (Melachim I, 10:22).
Radak: If it refers to Shlomo, all kings used to give gifts to him. Also if it refers to Mashi'ach, clearly it will be so. "Yashivu" teaches that they bring to him every year, i.e. time after time - "v'Heshiv l'Melech Yisrael Me'ah Elef Karim" (Melachim II, 3:4), "Heshivu Eshkarech" (Yechezkel 27:15).
Malbim (8, 10): Via doing Mishpat, he will merit great wealth. Kings of Tarshish and islands will give to him gifts, time after time.
Who are "Malchei Sheva"?
Rashi: This is Malkas Sheva 1 .
Refer to Melachim I, 10:1:1:1-2 and the note there.
What is "Eshkar"?
Rashi: It is a gift 1 .
Why does Radak say that nothing resembles this in all of Tanach?! He himself cited "Heshivu Eshkarech" (Yechezkel 27:15), and he explained there that it is a gift, like it means here! (PF)