If a magazine goes out of business, any manuscripts they have purchased the rights to still belong to them and they are free to pass them along to or sell them to the next owner. However, if the magazine is not picked up by a new owner, the rights to those manuscripts still belong to the original buyer. If you signed a contract on the sale of those rights, it should indicate what happens to those rights, or when/how they revert to you. If you sold all rights to the piece, you cannot resell it without asking the original buyer to release those rights to you. The problem may be that it is often difficult to find out who owns the rights if the publication has gone out of business. Even if you sold only first rights, the purchaser still has the right to publish the material the first time, so you are still responsible to get permission, or get the buyer to release those rights to you. However, if you sold them only one-time rights or reprint rights, you may continue to sell it elsewhere without seeking permission from the original buyer.