I apologize if I won’t answer all your questions, too many, but since you seem to be young and full of good will I’ll share my 2c. worth opinions on the matter. Smartphones booming era is past, that’s clear. I read a couple of days ago a report from https://www.bankmycell.com/ on smartphones value loss and the numbers are impressive, we’re living a deflationary time never seen before when phones value decreases a percentage point every 3/4 days with iPhones, much worse for Samsung and other cheaper Android alternatives.
eBay is essential for spares and repairs parts and there’s a huge offer..as you can see for yourself here in Europe at any given time there are up for sale >1500 devices. Most are just dreamers who hope a broken old phone is still worth something close to what big chinese sellers ask for a working one..heaven only knows why. Then you have rubbish diggers, they pick phones from anywhere and never tell you what you are going to buy, you may pick a bargain but often the only one making a good deal is the seller. Finally you have scammers, they are usually the worst as they are well aware of what they are selling cause they’re in the business one way or the other but just tell you what you want to hear. These are often local resellers and repair shops and labs dropping on ebay phones in very bad conditions and rubbish they already half destroyed trying to hopelessly learn microsoldering basics. If you leave out all these bad deals, you’re left with a handful of sellers of phones that are truly fixable with a profit, private owners and a few serious sellers in the whole Europe, representing maybe 5 to 10% of the listings. Can you make a regular and profitable business out of that ? Consider carefully time involved in searching good deals, finding spares, repairing, finding buyers and deal with your own mistakes and then give yourself an honest answer.
Take also into account the fact that for the last couple of years we have had in EU very big chinese second hand phones dealers who seem to have no trouble to undercut prices for an iPhone 7 128GB down close to 200€ when they need to..do you expect you can you compete with them and make a profit ?
Last but not least, repairing iPhones has changed quite a bit. I’ve started with iPhones 5, done some 4 and 4s, and enjoyed relatively easy repairs until the 6s. From iPhone 7 onwards things have become seriously steeper. Circuit complexity has increased dramatically and faults have become more difficult to spot as many simple issues just cause bootloops. iPhone X increased difficulty further and again, as it needs special expensive tools just to start with. Is it worth to get into all this right now ? Not sure I’d recommend that to my son honestly but as I said, it’s just my 2c.