I've operated a small online business in the past selling bicycle parts so I knew a bit about online marketing, website creation etc. With that business I put myself through university and grad school with zero debt and money in the bank. I ended that business after my suppliers dried up.
Since I had extra time over Covid I decided to create a business pretty much exactly as you have described using many of the markups and services you already mentioned. I secured 50 pixel phones from a reputable wholesale refurbisher. The phones looked brand new.
I created a marketing and information / privacy tutorial website, created a Youtube presence with various phone and setup tutorials, set up a social media presence etc. Some of my Youtube videos received over 35,000 views. Initially I was only asking a top up of $125 for what I paid for the phones and what those same models (in worst shape) would be selling for on eBay. This was all before the GOS web installer worked well (IMO). Sold a few but sales were not great. The customers I did have were very satisfied and said I offered the best customer service they've experienced in a long time.
Every month that went by, the end of life date for updates for the phones came closer. Eventually I gave up and sold them on eBay for maybe a $25 to $35 markup (after fees). I'm a seller on there with a 1000+ feedbacks and a 100% satisfaction rating. There were other guys selling on eBay at the time for more money but I never saw their phones moving much. Not sure what happened to them.
After all the work I did to set this up, it would of been better for me to to get a second job delivering pizzas in the evening. With the profit I made and the hours I put in, it was maybe $1.25 / hr.
I'm not saying don't do it but unless you have good marketing knowledge, are willing to risk some money, can identify and target a ready audience, and want to play a long game, then I would say there are easier ways to make money.