Both verbs are present tense, they are weeping and they are mourning, as if John is seeing them as they weep.
for no one buys their merchandise
Merchandise is γόμον [gomon] , indicating a cargo or freight load of goods.1 Not only was Babylon a great consumer of luxury goods, she also served as a center of trade. Commercialism is a key contributor to the materialism and godlessness which characterize the city at the end. Although material goods are not inherently evil, an abundance of material wealth often contributes to covetousness and idolatry. As people turn their attention increasingly towards making money and obtaining goods, they neglect the more important things of God. In her destruction, God will destroy the idols of commercialism and materialism. Tyre had suffered a similar judgment (Eze. Eze. 27:27). The merchants are like the church at Laodicea, whom the Lord threatened to vomit out of His mouth because you say, I am rich and have become wealthy, and have need of nothingand do not know that you are wretched, miserable, poor, blind, and naked (Rev. Rev. 3:17+). See commentary on Revelation 3:17.
Happy is the man who finds wisdom, and the man who gains understanding; for her proceeds are better than the profits of silver, and her gain than fine gold. She is more precious than rubies, and all the things you may desire cannot compare with her. Length of days is in her right hand, in her left hand riches and honor. Her ways are ways of pleasantness, and all her paths are peace. She is a tree of life to those who take hold of her, and happy are all who retain her. (Pr. Pr. 3:13-18)
1 James Swanson, Dictionary of Biblical Languages With Semantic Domains : Greek (New Testament), electronic ed. (Oak Harbor, WA: Logos Research Systems, 1997), G1117.