Gymboree experts, I need your help

[Saartje]

I have three turtleneck onesies (6-12 month size) that were given to us before DS was born. I washed them when he grew into that size, but they seem to run a bit small (or else I've been spoiled by HA sizing, lol), and they quit fitting by the time the weather cooled off. I'd love to sell them, but I've never even been to a Gymboree, so I have no idea what these would have cost new or how much they might go for used. Any tips?