How can I setup an item so I can sell by the piece or buy the case? And keep up with inventory?

Good day, shawnb1.

I'll help you set up the item but before doing so, let's make sure to enable the unit of measure in your preferences.

Here are the steps:

1. Go to Edit > Preferences.
2. On the left panel, select Items & Inventory, and click Company Preferences.
3. Under UNIT OF MEASURE section, click the Enable button.
4. Hit the OK button.
5. Select the unit of measure mode that is right for your business.
6. Click Finish and OK.

Please see this screenshot for a visual guide:

Once done, you can set up the item by following these steps:

1. Go to List > Item List.
2. Click the Item drop-down located at the bottom, and select New.
3. Choose the Type from the drop-down list and enter the item information.
4. Select Add New from the U/M Set drop-down list.
5. In the Select a Unit of Measure Type, choose Count (each, box, case, dozen, etc) and click Next.
6. Select Each and then click Next.
7. Put a check mark on the Case option and click Next.
8. Enter the number of each per case and select OK. Then, click Next.
9. Select the default unit of measure and select Next.
10. Set the name, and click Finish. Then, hit OK once done.

Here's a sample screenshot for reference: 

The following article includes more information: Use single and multiple units of measure for items.

Feel free to comment on this thread if you have any additional questions or concerns. Thanks.