Sometimes cheaper to do it twice. I'm planning to get the 3600 for about £190 then in a few years time I'll upgrade it to a 3900X when they're obsolete for around £100. I dont need the extra cores right now in 2019 but it will be a good upgrade path in years to come.
Dunno, even the 1800X isn't £100 second hand. Been way over 2 years now.