code: public void getTPTransaction(String API, Transaction.Time time, Transaction.Type type, Callback<List<Transaction>> callback) throws GuildWars2Exception, NullPointerException {
  		isParamValid(new ParamChecker(ParamType.API, API));
  		if (time == null || type == null)
  			throw new GuildWars2Exception(ErrorCode.TransTime, "Transaction time/type cannot be empty");
  		gw2API.getTPTransaction(time.getValue(), type.getValue(), API).enqueue(callback);
  	}
docs: For more info on transactions API go <a href="https://wiki.guildwars2.com/wiki/API:2/commerce/transactions">here</a><br/>
  Give user the access to {@link Callback#onResponse(Call, Response)} and {@link Callback#onFailure(Call, Throwable)} methods for custom interactions
  
  @param API      API key
  @param time     current | History
  @param type     buy | sell
  @param callback callback that is going to be used for {@link Call#enqueue(Callback)}
  @throws GuildWars2Exception  invalid API key
  @throws NullPointerException if given {@link Callback} is empty
  @see Transaction transaction info