How do people sell solids? (Classic)

I have a solid that I've been hanging on to, but my membership is ending soon and I don't plan on renewing it for awhile. I bought the membership for a month during a nostalgia rush, but I think I'm done with it now. I've seen people selling their solids for 750-800 diamonds, but I noticed the max diamond price in shops is 500. So how do they do it? I assume they sell multiple items to make up the price. Is that safe? Is there a better way to sell it?