You currently have this relationship:
Buyers::BuyerID = EQT::BuyerID
I'm guessing here that you are using FileMaker 11. That gives us a fairly simple option for this.
Add this relationship to what you already have:
Buyers::BuyerID X AllBuyers::BuyerID
In Manage | Database | Relationships, select Buyers, then click the button with two green plus signs to make a new table occurrence of Buyers and name it AllBuyers. Drag from BuyerID in one box to BuyerID in the other, then double click the new relationship line to pop up a dialog where you can change = to X. The X operater means that any record in Buyers will match to every record in AllBuyers.
In the same fashion, add a new table occurrence of EQT, AllBuyers_EQT and link it to AllBuyers by EquipmentID.
AllBuyers::BuyerID = AllBuyers_EQT::BuyerID
Define two text fields in Buyers as Brand and EQTtype.
In layout mode, add a portal to AllBuyers on your Buyers layout. In the portal setup... dialog that pops up, click the Portal filter check box and enter this filter expression:
( AllBuyers_EQT::EquipmentType = Buyers::EQTtype ) or ( AllBuyers_EQT::Brand = Buyers::Brand )
Almost done, now write a one line script to refresh the portal every time you select or enter a different Brand or Equipment Type: Refresh Window [Flush Cached join results]
Format your Buyers::Brand and Buyers::EQTtype fields with value lists of Brand and equipment types from your EQT table and use either an OnObjectModify (pop up menu, check box, radio button) or OnObjectExit (Drop down list) script trigger to run the Above one line script.
This is a fairly simple approach just to get you started. More sophisticated portal filters that match on different rules are also possible.