The cheapest line does the most damage. A customer who cannot get one size leaves without the other nine items too, so availability across the range matters more here than the value of any single product.
Loose selling erodes the count. Six taken from a box of five hundred is rarely recorded properly, and after a year the system and the drawer disagree enough that reordering becomes guesswork.
Trade and retail are two price lists. Agreed rates, credit limits and monthly invoicing for accounts have to run alongside cash sales without either interfering with the other.
Specials take as long as they take. Something ordered from a manufacturer arrives on their schedule, and a promise made at the counter without checking that lead time is the most common broken promise in the trade.