Many charity shops allocate a week number to each of their books.
So, since the first full week of 2014 ( = Week 1) started on Monday 6 January, the current week would be 'Week 6' which would be pencilled inside the cover of any books put out on display this week. Depending upon stock availability, and upon whether there are other shops with which books can be interchanged, books might be left on the shelves for anywhere between 3 and 12 weeks. But, as an example, some shops might operate a policy whereby all remaining 'Week 1' books on the shelves are removed during 'Week 6', with 'Week 2' books being taken off the shelves next week (i.e. a gap of 5 weeks between the 'on' and 'off' numbers').
Shop using coloured stickers are doing the same thing but they'll simply have a code such as this:
Week 1 = Red
Week 2 = Blue
Week 3 = Green
Week 4 = Yellow
Week 5 = White
Week 6 = Black
So, based upon the model above, they'd be putting black stickers on any books put out this week and removing any books with red stickers. Next week red stickers would come back into use for new acquisitions as it would be 'Week 1' again.
Of course there has to be a policy as to what should happen to books that are removed from the shelves. Some shops might offer them immediately at half price, whereas others (with sufficient storage space) might allow unsold books to pile up until they can all be offered in a special book event later in the year. Other shops (with no shortage of donations) might simply choose to send unsold books for pulping (but possibly holding some of the better ones back to be offered to local secondhand book shops in bulk at a later date).
Policies need to be worked out individually for each shop as they will be affected by:
(a) the rate at which donations are received ;
(b) the volume of sales achieved ;
(c) the amount of display space available for books within the shop ; and
(d) the amount of storage space available elsewhere for unsold books.