Inventory

tl;dr Keeping commonly used parts in inventory will ensure you don't lose customers.

There are three basic reasons for keeping an inventory:

1. Time: If a new customer asks for a repair and you respond, "I'll have it fixed in 5 to 7 business days," that customer will look elsewhere. Keep enough parts in supply at least 2 weeks of common repair.
2. Uncertainty: If your supplier runs out or increases prices, your business will suffer in the short term as you look for new supply.
3. Economies of scale: The ideal condition of "one unit at a time at a place where a user needs it, when he needs it" tends to incur lots of costs in terms of logistics. So bulk purchasing, shipping, and storing brings in economies of scale. This results in lower costs to your business.

A few guidelines for building your inventory:

• Always test parts before adding them to inventory.
• Keep parts in ESD safe baggies.
• Use a revolving inventory.
• Overstock commonly used parts, like display assemblies.
• Understock parts that lose value, like batteries.
• As your business grows, upscale your inventory by stocking a wider range of parts.

A warning:

Don't buy too much inventory. Online stores can always get rid of overstock by putting the items on sale. If you have 100 iPhone 4 earpiece speakers too many, it isn't so simple as just putting the repair on sale. You will always be limited by the demands of your local market. Always keep that in mind.