I just bought 2x sony experia xa2's off ebay for £15each - so as I have some back ups for if i lose my other one. Unlockable bootloader so long as sony keeps its developer website thingy open. lineage os (dependant on above) . headphone port. micro-sd card slot. decent size. I don't see any downsides - i don't understand why people get rid of em. Maybe they don't know about lineage OS.
£300 for a fucking phone ? good on you if you can afford that.