As comics-scifi-collectibles pointed out, eBay still has some work to do converting all the content and links across the website, and your browser is warning on that.
The main page address (URL) HTML does not have to be what is considered insecure - any image, script, iframe, whatever that is loaded in the course of loading that page and is not HTTPS content is targeted by newer browsers (mixed content).
There could be a single 1x1 pixel tracking gif loading on that page that has an HTTP (not HTTPS) URL, and the browser will throw a fit.
Theoretically, in the strictest academic sense, the page is insecure.
In real life, even in the unlikely event a very sophisticated hacker decided to target you to gain access to an eBay account, and was able to somehow leverage that single insecure entry point via a man in the middle attack (insecure WiFi or pwning the user's ISP or another upstream) where the attacker is able to intercept traffic and modify the single unencrypted, insecure, image to somehow gain further access (cookie stealing? other exploits?), the likelihood of a successful exploit is low, and the likelihood of anyone going to that effort is even lower (they have better luck and effort/reward ratios with phishing emails, keyloggers, and utilizing credential from other data breaches.)
Once again, I'm not saying that in the strictest sense what you are seeing is not an actual insecure page, but I consider it to be a non-issue, and the odds of it being a problem are close to zero (there are more likely exploits and compromise modes for users than that.
And keep in mind that up until a year or two ago, most websites had mixed content, and it was a non-issue until a couple of research whitepapers came out with theoretical exploit pathways, and Google jumped on it and decided to secure the internet all on their own and force everyone to comply (because of their huge influence and Android and Chrome userbase)
Someday (soon?) eBay will get everything transitioned and that warning will go away.
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Also want to point out for everyone else in general that folks with mobile site or mobile app issues need to post in the correct forums (for example the eBay mobile app forums for mobile app problems where there are eBay employees and knowledgeable users to help), and if posting in the general forums (like this Technical Issues forum), state that your problem is mobile related.
In spite of the fact that eBay and others claim 60% or more of online shopping is via mobile, blah, blah, blah, the baseline assumption (here and almost everywhere except mobile specific forums) is that when an issue is posted, is that it is a main website, desktop browser related problem, and as in this thread, where it was not mentioned that the issue was occurring on a mobile device until a ways in, a lot of time and effort was wasted looking at the problem not knowing it was mobile
This is why I may seem cranky with mobile users - mobile problems are total apples and oranges to main website issues and it's non-productive and frustrating for most of us to look into them and post desktop browser links, workarounds, and screenshots that are not applicable because we don't know it's a mobile issue. (some users here do use mobile and would have some hints or input knowing it is for instance, and iPhone/Safari browser issue, but they abstain thinking it is a desktop issue because insufficient info was provided in the OP)
(Don't fret OP, I just considered this a good opportunity to mention this here, and in your case, it isn't a total mobile only problem although it only appears in mobile context. )
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I assume that's an Android phone in the screenshots, and by all devices you mean an Android phone, and other Android phones and tablets?
I don't have any Apple devices at all, or any current Android devices that would be able to replicate what you are seeing. I don't have an Android emulator set up that works (the couple I tried in the past did not do what I needed them to, and it appears to more of a PITA than I'm willing to go to find one or pay for one that does work well - I'm not a developer)
My Opera Mini emulator (that does lots of Android devices) is too old, no longer supported or updated, and does not complain about the pages you are seeing the problem on (it is from before browsers started warning on mixed content)
My Windows phone using Edge or Opera Mini has no issue with the pages either - and in any case, even if it did, I have next to no diagnostic tools on the phone - it's about as locked down as an iPhone - so I wouldn't be able to investigate anyway.
In light of all that, I can't help you any more with specifics or narrowing down what exactly might be triggering the browser warning. eBay may be dishing up slightly different content to a mobile Android device, but I can't emulate that or replicate it.
Your best bet would be to take the question to the Android mobile app forum and see if anyone else sees what you do, and see if the eBay helpers there can determine what may be happening, and maybe refer it over for a fix if warranted. https://community.ebay.com/t5/Mobile-App-Android/bd-p/androidapp
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