I think they are worth keeping, I've used my stockers a few times. Just switch them out when I need to do a good cleaning on the 18's. I've been using them all this week. Probly still use them till the 24th, big show on the 25th. Right now my 18's are nice and clean.
You won't get what you think they are worth, they are worth more when you are in need of some wheels for any reason.
Loaded down camping! My 18's would rub with the 15offset (25mm spacer)