In the past couple years, I bought several sets of lightly used flats that were cut for 4-in-line fender-type headstocks (like my Lakland). I trusted the sellers, all TBers, that they were lightly used. I wiped the strings down when I got them, just in case... I put them on, tried them for a few hours, and typically sold them for around what I paid. This allowed me to audition several kinds of flats without breaking the bank (and I didn’t catch any nasty diseases either). I’d personally never keep them on for a long time, due to concerns regarding their integrity... I’ll get a new set when I decide to switch to flats (I’m on pressurewounds now), and know exactly what set I want.
Rounds, that’s a different story... they get crud in there, and they’re rather cheap new so there’s not much room for cost saving. That throws the cost-benefit ratio way off imo... I do keep my old ones, just in case, as long as they’re not completely dead... sometimes I want to do a quick AB test, or just have a backup that’s aged (I like my rounds a bit old too).