You should try to offer an item (or items) that are similar in value to the item(s) being auctioned. Alternatively, you can offer a combination of money and items. Keep in mind that your swap offer may be rejected if the seller doesn't feel your swap is of equal value with his or her auction, or if the seller doesn't have a need for the item(s) you are offering. Don't take it personally if your offer is rejected, it's possible that the seller is only looking for a specific list of things that your item or items didn't happen to be on.