A merchant service

Merchant.java

package org.spongepowered.api.service.merchant;

import org.spongepowered.api.CatalogType;
import org.spongepowered.api.service.economy.account.Account;

import java.util.Collection;
import java.util.Optional;
import java.util.UUID;

public interface Merchant extends CatalogType {

    Collection<OfferType<?>> getSupportedOfferTypes();

    Collection<Offer<?>> getOffers();

    Optional<Offer<?>> getOffer(UUID uniqueId);

    <T> OfferTransactionResult<T> buy(Account buyer, Offer<T> offer);

    <T> OfferTransactionResult<T> sell(Account seller, Offer<T> offer);
}

Offer.java

package org.spongepowered.api.service.merchant;

import org.spongepowered.api.Nameable;
import org.spongepowered.api.Sponge;
import org.spongepowered.api.service.economy.Currency;
import org.spongepowered.api.util.Identifiable;
import org.spongepowered.api.util.ResettableBuilder;

import java.math.BigDecimal;
import java.util.Optional;

public interface Offer<T> extends Identifiable, Nameable {

    @SuppressWarnings("unchecked")
    static <T> Builder<T> builder() {
        return Sponge.getRegistry().createBuilder(Builder.class);
    }

    Currency getCurrency();

    BigDecimal getPrice();

    OfferType<T> getType();

    Optional<T> getValue();

    interface Builder<T> extends ResettableBuilder<Offer<T>, Builder<T>> {

        Builder<T> name(String name);

        Builder<T> type(OfferType<T> offer);

        Builder<T> value(T value);

        Offer<T> build();
    }
}

OfferTransactionResult.java

package org.spongepowered.api.service.merchant;

import org.spongepowered.api.Sponge;
import org.spongepowered.api.service.economy.transaction.TransactionResult;
import org.spongepowered.api.util.ResettableBuilder;

public interface OfferTransactionResult<T> {

    @SuppressWarnings("unchecked")
    static <T> Builder<T> builder() {
        return Sponge.getRegistry().createBuilder(Builder.class);
    }

    Offer<T> getOffer();

    TransactionResult getResult();

    interface Builder<T> extends ResettableBuilder<OfferTransactionResult<T>, Builder<T>> {

        Builder<T> offer(Offer<T> offer);

        Builder<T> result(TransactionResult result);

        OfferTransactionResult<T> build();
    }
}

OfferType.java

package org.spongepowered.api.service.merchant;

import org.spongepowered.api.CatalogType;

public interface OfferType<T> extends CatalogType {

    Class<T> getAllowedType();
}

OfferTypes.java

package org.spongepowered.api.service.merchant;

import org.spongepowered.api.item.inventory.ItemStack;
import org.spongepowered.api.util.generator.dummy.DummyObjectProvider;

public class OfferTypes {

    public static final OfferType<ItemStack> ITEM = DummyObjectProvider.createExtendedFor(OfferType.class, "ITEM");

    public static final OfferType<String> COMMAND = DummyObjectProvider.createExtendedFor(OfferType.class, "COMMAND");
}