Just to listen to the old essentials. Partly to find the tracks I've forgotten. re ebay, it can be good, but it can be nuts. I saw something sell on ebay for 3 times its better condition single listing on discogs. Crazy! Apparently car boot sales are the prime place to pick up a few stacks of vinyl, if you get it at pennies on the disc, working through and filtering the good out could be a bit of fun. There's a pioneer system out there that loads in mp3 files and so can play through a library of stuff, and it counts the bpm for you and can visually sync up beats. It's less than a tenth of the original skillset, but validating in its way.
So far as my mixing goes, I needed to keep it up in a former life, because working backwards is very difficult. More just for fun now. When I got back into it, I made a promise that I'd treat myself to a couple of discs a month. That was doable until circumstances changed, but like you say there's bargains to be had on ebay if you look and search and research and catalogue. Sometimes a punt on a job lot worked out.