function buyByUsdt(uint256 tokenId) public {
    require(usdtPrices[tokenId].price > 0 && erc721Address.getApproved(tokenId) == address(this));
    require(usdtToken.transferFrom(msg.sender, usdtPrices[tokenId].tokenOwner, usdtPrices[tokenId].price));

    erc721Address.transferFrom(usdtPrices[tokenId].tokenOwner, msg.sender, tokenId);
    resetPrice(tokenId);

  }