z201205 it should be removed from the website as soon as possible
In the case of refurbished phones, giffgaff isn't going to know whether buyers will return a particular phone. If statistically 3% of phones get returned and they've sold a hundred of them this month, they can probably predict getting three of them back. But they might get none or six. OK, so they can keep it listed and there may be one the following month. But what if they only sell 10 per month? Then they'll probably not have another to sell for 3 months, but if they remove it from the listing, chances are one will come in the next day.
It might be easier with new phones, but even there I suspect the best bargains come from buying up overstock which is probably equally unpredictable.
The way it's done with other types of products is to implement a filter "only show products in stock". giffgaff doesn't even have that, but I suspect it would be useful.