A few suggestions

[Gershon123]

Hi there, I forked your plugin and it's working really well. I have a couple of suggestions that I implemented that would improve this plugin.

Here is a gif:
https://i.imgur.com/U5FRm9j.mp4

• Replace "Buy & exit" & "Buy and back" with just "Back" & "Buy", if a user want to buy and exit they can just press Esc after buying something
• Show the quantity of the item in the menu. This will restrict to buy 64 at a time, but I'm okay with that.
• Replace quantity options to "Set to 1", "Remove 1", "Remove 10", "Add 1", "Add 10", "Set to 64". This improved the quantity experience, my users were confused with the original
• Remove the "Clear" option since we auto start at 1 time, so that option is not needed
• Hide "Sell" when you can't sell an item
• Add a /sellhand command, this allows users to not need to open the shop (I will provided a snippet)
• Replace the shoplist in the config with just loading shops by their file name, this resolved some of the saving issues I had and others reported (I will provide a snippet)

I would create a pull request but I'm running off your API 7 fork, and I'm not sure if you're okay with some of my changes.

Snippets:
Sellhand

package sawfowl.guishopmanager.utils.commands;

import org.spongepowered.api.command.CommandException;
import org.spongepowered.api.command.CommandResult;
import org.spongepowered.api.command.CommandSource;
import org.spongepowered.api.command.args.CommandContext;
import org.spongepowered.api.command.spec.CommandExecutor;
import org.spongepowered.api.data.type.HandTypes;
import org.spongepowered.api.entity.living.player.Player;
import org.spongepowered.api.item.inventory.ItemStack;
import org.spongepowered.api.item.inventory.query.QueryOperationTypes;
import org.spongepowered.api.scheduler.Task;
import org.spongepowered.api.text.serializer.TextSerializers;
import sawfowl.guishopmanager.GuiShopManager;

import java.util.Optional;
import java.util.concurrent.atomic.AtomicBoolean;

public class SellHand implements CommandExecutor {

    GuiShopManager plugin;

    public SellHand(GuiShopManager instance) {
        plugin = instance;
    }

    @Override
    public CommandResult execute(CommandSource src, CommandContext args) throws CommandException {
        if (!(src instanceof Player)) {
            src.sendMessage(plugin.getLocales().getLocalizedText(src.getLocale(), "Messages", "OnlyPlayer"));
        } else {
            Player player = (Player) src;
            Optional<ItemStack> itemStackOptional = player.getItemInHand(HandTypes.MAIN_HAND);
            AtomicBoolean foundItem = new AtomicBoolean(false);
            if (itemStackOptional.isPresent()) {
                ItemStack itemStack = itemStackOptional.get();
                plugin.getAllShops().forEach(shop -> {
                    shop.getMenus().values().forEach(shopMenuData -> {
                        shopMenuData.getItems().values().forEach(shopItem -> {
                            if (shopItem.isSell() && itemStackEqualsIgnoreSize(shopItem.getItemStack(), itemStack)) {
                                foundItem.set(true);
                                Optional<ItemStack> playerHand = player.getItemInHand(HandTypes.MAIN_HAND);
                                if (playerHand.isPresent() && playerHand.get().equalTo(itemStack)) {
                                    player.setItemInHand(HandTypes.MAIN_HAND, null);
                                    shopItem.getPrices().forEach(serializedShopPrice -> {
                                        Task.builder().async().execute(() -> {
                                            plugin.getEconomy().addToPlayerBalance(player, serializedShopPrice.getCurrency(), serializedShopPrice.getSellPrice(), itemStack);
                                        }).submit(plugin);
                                    });
                                } else {
                                    player.sendMessage(TextSerializers.FORMATTING_CODE.deserialize("&cSale cancelled."));
                                }
                            }
                        });
                    });
                });
            }
            if (!foundItem.get()) {
                player.sendMessage(TextSerializers.FORMATTING_CODE.deserialize("&cThis item is not sold."));
            }
        }
        return CommandResult.success();
    }

    public static boolean itemStackEqualsIgnoreSize(ItemStack o1, ItemStack o2) {

        ItemStack copy1 = o1.copy();
        ItemStack copy2 = o2.copy();

        copy1.setQuantity(1);
        copy2.setQuantity(1);

        return copy1.equalTo(copy2);
    }

}

Loading shops as files

    @Override
    public void loadShops() {
        Task.builder().async().execute(() -> {
            try {
                File[] files = plugin.getConfigDir().resolve(plugin.getRootNode().getNode("StorageFolder").getString()).toFile().listFiles();
                for (File file : files) {
                    try {
                        ConfigurationLoader<CommentedConfigurationNode> shopConfigLoader = HoconConfigurationLoader.builder().setPath(file.toPath()).build();
                        CommentedConfigurationNode shopNode = shopConfigLoader.load();
                        String fname = file.getName();
                        int pos = fname.lastIndexOf(".");
                        if (pos > 0) {
                            fname = fname.substring(0, pos);
                        }
                        plugin.addShop(fname, shopNode.getNode("ShopData").getValue(TypeToken.of(SerializedShop.class)).deserialize());
                    } catch (Exception e) {
                        plugin.getLogger().error("Failed to load " + file.getName());
                        plugin.getLogger().error(e.getLocalizedMessage());
                    }
                }
            } catch (Exception e) {
                plugin.getLogger().error(e.getLocalizedMessage());
            }
        }).submit(plugin);
    }