The ALPG allows players to sell gear for half its listed value. PCs can purchase/sell gear before the start of an adventure, or between chapters/episodes of a hardcover adventure. The admins have clarified that this rule allows PCs to sell gear even before the start of their first adventure. I am one of the former regional coordinators and spent considerable time answering questions like this every day on FB, but you don't need to take my word for it. Feel free to contact the admins on the official D&D AL facebook groups if you would like them to answer your question in person.
I believe you. Since I just started with the AL, I only had the 5.1 version of the players guide. 5.1 decided to delete the section that contained this rule. Going back and reading all the other versions I was able to see it. I am wondering though if removing the rule was intentional to get rid of it. Like the change from 1.0 to 2.0 that removed using starting gold and only allowed the starting equipment.