I've sold at least a dozen used snowboards to private parties. I try to sell snowboards locally on craigslist and facebook marketplace to avoid shipping. I've sold a TON of other outdoor gear, shipped across the US. Your question has multiple parts:
BOXING:
I try to find a used snowboard box from a local shop. Since it's the off season here in the Northern Hemisphere, that might be tough. I purchased a new splitboard from Cardiff last season, and they sent it in a FLAT box folded in half! It was interesting to receive a flat box from UPS with a powder early rise end to it. So that's a possibility. Wrap the shit out of it in bubble wrap, then cut up a bike box that you can easily get from any bike store. Use clear packing tape and NOT duct tape.
SHIPPING:
UPS or FedEx GROUND. Will be expensive unless you know someone with a commercial account. Go directly to UPS or FedEx, and NOT a UPS shipping STORE. This will be a little cheaper. You can ask the buyer to kick in an extra $20 to help offset shipping costs, OR I usually bump up the price of the item and cover shipping.
SELLING:
Start an email chain with the buyer. Get their name, mailing address and phone number. Confirm the phone with a text. I ONLY deal in Venmo (sorry Canada). Once payment is received, transfer it into you linked account THEN ship the item. Take a photo of the box with the shipping label on it. Email the buyer photos of 1) the box and 2) the shipping receipt with tracking info.