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If you're going to sell them, stripping them down first would be a solid idea. Pretty easy to do.
What is your recommended method
Biostrip 20 would be my recommendation, useable on both metal and plastic (resin is a bit iffy). It's non-toxic (and won't eat through your flesh to the bone like some of the more brutal stuff on the market) and also doesn't really smell which is usually another issue with paint strippers. There's been a few threads on here about it
These days I dump mine in Dettol for a few days. Quick scrub occasionally and sorted.
If you end up putting them on Ebay, maybe stagger them a bit as if you put them all up at once someone will report you as a recaster!