SorterC.cs

using System;
using System.Collections.Generic;
using System.Linq;

namespace MarketSort.Solutions
{
    class SorterC : ISorter
    {
        public IEnumerable<string> Sort(IEnumerable<string> values)
        {
            return values
                .ToDictionary(key =>
                {
                    var split = key.Split('-');
                    return split.Length == 1
                    ? (-1, -1)
                    : (
                        Convert.ToInt32(split[0]),
                        Convert.ToInt32(split[1])
                    );
                }, val => val)
                .OrderBy(x => x.Key.Item1)
                .ThenBy(x => x.Key.Item2)
                .Select(x => x.Value);
        }
    }
}