You are the PM of Facebook Buy/Sell Groups. Engagement on listings in these groups is trending up, but the total number of listings has plateaued. Why?

Clarifying question:

1. What does engagement on listing is treding? Do we know which  key user action is driving engagement? Assume '% DAU/MAU users viweing listings' is increasing

2.Are we talking about all listing or listing for specific category(like shoes, watch etc) getting plateaued? Assume its across categories 

3. Are we seeing this trend for the 1st time or its seasonal /holiday season? Assuming its not seasonal

4. Is this an sudden or Gradual change? Assume Gradual.

Product Info: Buy/Sell group is a Group specifically working as a Closed Marketplace for sellers and buyers with similar interest based on geography, commodoty category type, new vs old etc. Seller add a listing to this group and other sellers n buyers would be able to see that listing based on Private or Public settings configured for the Group.

User Journey:

There are 7 key actions from the buyer's end: Search, Like,Send Message,  Save, Send Offer, Share listings, Final Purchase(Payments). 

There are 3 key action from seller's end : Add a Listing, Post it to Group, Respond to a Message ,Quote or Purchase Order, Accept Payment.

Hypothesis:

1. New and Existing Buyers are more active than sellers , so engagement may still go up while listing can remain intact.

a. New Marketing campaignor promotions focusing buyers.

b.New product/feature launch which spread more awareness among buyers to use Buy/Sell Groups

c. New Payment method are introduced like Paypal, venmo etc

d. Discounts on existing listings given by sellers

2. Seller are facing issues with new listings

a. UX/UI issue not allowing to add, post or complete a listing

b. Potential Bug making some optional fields as mandatory which is leading to drop-off in adding a listing. 

c.Higher Payment Processing fees on newer listings

d. Lesser new sellers meaning lesser new listings

e.Lesser inventory with existing sellers meaning lesser new listings

f. A new Complaince, tax or regulatory impact for sellers.

g.New Listings are brutally reviwed by FB before showing it as listing because of complaints of counterfeit and fraudsters.

I would like to do some data analysis for each of my hypothesis and funnel it down to top 2-3  hypothesis which would prove the actual reason. I would fix them individually and run A/B test to see impact on the engagement as well as listing. 

Finally will recommend which of the A/B testing made the biggest impact and scale it accordingly and sequentially