I used to manage a private day nursery and was in charge of ordering and have worked in several others.
It's the industry standard for private day nurseries to pay as they order as basically there aren't special rates for nurseries. Although there are specialist day nursery suppliers for equipment, toys etc they will pay the same £80 for a travel cot for example, as the customer on the high street in Mothercare.
Nurseries attached to schools and nurseries run by Sure Start/Local Authority would probably expect 30 days credit though. You may find that the big national chain nurseries ( Teddies, Leapfrog) will probably expect 30 days credit.
Hope this helps
Claire