The "merchants" to which this refers is in the context of the Gospel being by Grace through Faith. Those attempting to "purchase" their way to Heaven by their good works, are thus the "merchants". The Harlot (counterfeit "christianity") is a "market" selling all sorts of wares, and that which she sells is a works "gospel".
Salvation is not by works of men, but men are saved to walk righteously. Works are the result of justification not the means of it.
Demonic influences can enter into any endeavor of Man, including the Gospel Coalition. Being grounded in the Word, submitted to and in Christ Jesus the King by the Word of His Testimony, keeps one from being lost through any demonic influence.
9 The coming of the lawless one is according to the working of Satan, with all power, signs, and lying wonders,
10 and with all unrighteous deception among those who perish, because they did not receive the love of the truth, that they might be saved.
11 And for this reason God will send them strong delusion, that they should believe the lie,
12 that they all may be condemned who did not believe the truth but had pleasure in unrighteousness. 2 Thes. 2