advising against 26" is one of the worst things i've read on here.
used bikes are not that tricky, if you find something your size that looks good, research it and the retail/average sold price to see if it's a good deal or not. and when checking out used bikes in person, test ride, make sure everything works and can be adjusted, nothing rusted or seized, no dents on the frame, etc. i think it's easy to tell when a bike has been treated well. though i would probably go with whatever your parents have, especially if they just need tires/tubes.
Maybe.. I don't know what bikes you ride or have riddin in your lifetime. I just can share what I know from a mechanic perspective. I'm a trained professional with a decade of experience. Many people came to me with used bikes, i mostly talk about highend stuff, or what used to be high end. Starts with suspensions that have never seen service. Brakes that have never been bleeded, running on break disc's that are way over min. measurements. Chains and sprockets that "look" clean but have never been changed and are way over miles so you have to change everything in the near future. Spokes and rims that are fucked, even when they "look ok". Headsets that are watered but hey they turn. Bearings rusted into the frame, mostly common on fullys that got "cleaned" with a pressure cleaner, winter bikes and bike park bikes. The list goes on but whatever. Maby I'm just to picky because I'm a mechanic and have seen too many shity used bikes.
Mostly bought on ebay from people with no clue.
And if you wanna ride 26" man, go for it.. I've never seen a 26" in years besides kids bikes. Even 27.5 is kinda rare these days besides women sizes and mullet dirt bikes. Damn even my 70 year old dad rides a 29" pedelec these days.
I'm German btw, so all I refer to is the marked here. I have no clue what's going on elsewhere. All I know is that the US likes gravel bikes it seems. And I don't speak for anyone but myself. Maybe someone else has a different opinion or experiences.
I like 29" because it's fast (not in startup but after) and stable, more ground room and more traction and because I'm 186 cm big. It feels just nice.
27.5 is also great don't get me wrong. I think a mullet style bike could be something I would ride too. Best of both, but I haven't tested one yet.