Football loves its mantras. They get repeated so often that in time, they are treated as fact. If you had a penny for every time somebody said that there's no value in the winter transfer market, you could probably afford to buy Gareth Bale and a space shuttle to help him commute from Wales to Valdebabas in minutes.
I get the argument. The idea is that clubs are reluctant to part with their star players in January because finding a suitable replacement in mid-season is tough. Effectively, there's a buyers' chain. Your star will need to be replaced by a somewhat smaller club's star and they'll be reluctant to sell unless they can themselves find a replacement, too.