When I needed a new set of blades I order 2 sets, one for the customer & one for stock
As there used to be a 100% mark up on blades it meant I got a set for stock at n actual out of pocket costs.
Thus the blades I have in stock are the ones I need and the common ones like Murray MTD JD & AYP 42" I carry 10 sets , built up over time.
When a supplier does a run out of old slow moving blades I also buy some of them .
This practice has gotten me a quite a few customers because they go to the dealer & get told NLA then come to me & I have them in stock
So for instance I have a lot of old pattern MTD blades I bought at $ 5 & sell at $ 30 to $ 50 as the customers can only get them from online merchants in the USA .
Even then I get caught now & then as over the past 3 years of drought they have been eating blades .
Same story with belts so I always have customers belts in stock otherwise I loose the sale and for pats only customers, the customer as well .
I put a dot on the sleeve or some times the customers name so I know I am fitting a belt I must replace if I am doing a new customers mower.
Blades are one of those add on items used to get free delivery on small orders .
I try to have everything I need on hand so I can turn over the mowers quickly then replace the parts used in a monthly order.
Doing it this way makes it difficult for the auditors as a single repair may have parts ordered from 4 different suppliers anything up to 2 months after the job was done .
Modern accounting practice is to keep minimum inventory & order in only when needed so it confuses the baby faced tax men no end.
OTOH I do not pay tax on my inventory only on the turn over .