There are many phones out there that are easy enough to fix the common problems with the right tools (heat gun, opener tool, set of small screwdrivers and picks

There are many phones out there that are easy enough to fix the common problems with the right tools (heat gun, opener tool, set of small screwdrivers and picks). Replacing a dead screen is about an hour's work for a beginner and there are enough tutorials on youtube that most beginners will get it right first time. Most parts for common phones are commonly available on aliexpress. Most other problems can be dealt with by replacing motherboard/daughterboards or battery rather than needing to bother with reflowing solder. The real problem is that phones depreciate so quickly that replacement parts (especially displays - Samsung LED screens etc are notoriously expensive) are often more expensive than buying a new second-hand phone on eBay unless you're willing to go down the route of separating bonded LCDs from glass/touch sensors is economically viable but that is much more difficult to do right, even with the right equipment (temperature-controlled hotplate).