I need to link two 2 fields in one form.

One field would be read only but would populate as items from the second field are selected. Is this possible? The end result needs to be a form where the user can see items he has previously purchased in field one, but management would select those items in field two. If this is not possible is it possible to accomplish with two forms?

You can achieve this using the combine fields functionality using Update Field condition logic.

Check the user guide : How-to-Pass-Field-Value-to-Another-Field

If you need it between 2 forms, it can be done as well.

User guide: How-to-Automatically-Pass-Form-Data-to-Another-Form

For payment related process, then this one : How-to-Pass-a-Calculation-to-a-Payment-Field

Hope this help. Let us know if there is more we can assist you with.

Thanks.