Even if Lex sped up the meeting, he had to go through the necessary steps. He had to pick who he was trading with, and in the case of the emporium, he had to pick out exactly what he wanted to buy.
This was a rare opportunity to rack up on various resources, cultivation techniques and information that he wouldn't normally be able to get in touch with. This was one benefit of appearing as himself: he did not need to pretend to be all knowing and powerful.
Of course, the emporium could no doubt guess that Lex's background was not simple. But what did it matter if they guessed or not? Lex was still in the golden core realm. It was understandable for him to have gaps in his knowledge.
If he could supplement his knowledge with that of the emporiums, not only would he be better off, it would be a lot easier for him to maneuver in the future.
