Okay. We do this at our club a lot as an inperson swap, and things that help:
[Organize tables and have the skates arranged by size. Put a friendly but effective guard on the table to prevent the skates from walking away. Put a price on the skates themselves, and fasten pairs together.
Be prepared with printouts of current list prices on models so people know how much they WOULD be paying.
Get a bunch of experienced skaters to help out for fitting. Lots of parents don't know how to fit a pair of skates. It also cuts down on people damaging the skates by banging them around.
Set up a "fitting area" with carpet and chairs. You don't want people wrecking blades or floors.
You can also do a "blades only" area for people who have blades to sell but not boots.
I don't know how well an online sale would work. It's hard for people who don't know how to fit skates to buy them from a picture- ? And if they don't fit, you don't want to be caught in the middle of a "return due to poor fit" thing. But, then again, I've always been amazed at the things that people can and will buy online.