Call the number and tell the card number and pin for activation. The Walmart gift card can be used in-store, at Sam's Club, or online.
When choosing to buy a gift card on eBay, your biggest precaution would be to check the Seller's Feedback and makes it positive for Sales. https://groups.Google.com/g/giftcard-mall-my-gift-info/c/lJb4jNQqwjY