While browsing on SidelineSwap, you may come across a handful of items that do not have the "Buy Now" option that allows you to purchase the item for its asking price:
Some sellers manage their inventory in a way that requires them to check for the item's availability before committing to a sale. In order to purchase these items, simply make an offer for the asking price or less for the seller to review. Once the seller has received your offer, they'll check for the item's availability and accept or reject the offer.