Objective

Form a hypothesis to improve the usability of Letgo app. This would mean an increase of new user engagement and retention and possibly an increase in revenue.

Step 1. Before jumping into solutions and forming a hypothesis I need to find key UX pain points and solve the right problem first. So, using usability heuristics I will start from research.

Pain point #1: Onboarding process focus only on user who come to sell products. It is not benefit-focused (explains the 2-3 core benefits and how to achieve that benefit) neither function-focused (explains the 2-3 core functions of the product). All this can cause a drop-off of users, especially inexperienced ones. Competitors and similar products, such as Carousell, OLX, 5 miles, ebay etc. are more flexible with onboarding process. Examples of the first screen users see in different apps:

Pain point #2: Categories and filters are organised quite chaotic (with copying the categories) and with too annoying button "Sell your staff", accidentally users can tap it while scrolling.

Pain point #3: Listing item organised quite chaotic, some of the functions are copied or hided. It needs better structure and better prioritisation. For example first you see "Edit" button next to "Share" button, edit also hided in 3 dot and then it appears in "More info" but with different looking icon.
Problem with buttons weight and prioritisation: "Mark as Sold" looks more important then "Sell faster" motivation button which seems important for business. Map loos too important (fro user who are selling).
Also in Edit screen Title and Description are marked as optional but they have different colours. "Feature your listing to sell faster" switcher is turned on, but has no explanation and if you switch it off it's automatically turn on Share on Facebook.

Pain point #4: Auto-response buttons are very annoying and users are pressing them a lot accidentally with not real interest in a product. You can see that it annoys all users (sellers and buyers) through researching reviews even in Appstore. A lot of users are talking about this point:

Step 2. Prioritize.

Step 3. Turn problem into hypothesis

"How might we make people feel more comfortable when using the auto-messages?" Hypothesis: hide auto-message bubbles from the first screen.
We will know the hypothesis is valid if Net Promoter Score will increase. Or if it never measured before, it would be at least 45 (The average NPS for e-commerce is 45). This is a great way to get feedback because it's one question, low commitment, and customer has to take 0.2 seconds to respond to it, increasing chances for feedback. The best way to gather this feedback, is to show it after 3 sent/received messages.
Promoters - Detractors = NPS

Step 4. Prototype. As it doesn't need complex flow. I will start directly from designs. I will skip chart-flow, hand-drawing prototypes and low-fidelity prototypes.

You can check out Prototype here

Competitors benchmarks. Some examples from competitors (Carousell, Tradyo, 5 Miles, Blinker, Shpock, OLX):

I also checked OfferUp, StockX, Mercari, Depop, ThreadUp, Vinted, Poshmark, SocialSell. No one use quick reactions, but from what exist on the market, what use Instagram now seemed to me the most similar: