ID Number

Hi
I hope someone can help me please - I'm going round in circles on what should be the simplest of tasks but I cannot find a solution to it. Ive been through everything I can see in the forum to try to find an answer but I'm missing something. I'm pretty much a beginner for some concepts such as variables etc so go easy on me please:)

I have a main form for a product which has a serial number for each item - 1 item per record - and whenever I sell an item, I can open a new form to enter the buyers details.
As a buyer might purchase more than one item at a time Ive created a relational link to allow me to add several items to the buyers record via a sub-form in the buyer form and then print out a receipt.
All of this seems to work fine except that I want to be able to to see in the main form which transaction is associated with that particular item (to prevent me duplicating the sale) and also to search for a buyer or view a buyer by having a transaction ID on the main forms(s) which matches the same transaction ID on the buyer form.
I know it should be something like using a record ID for each buyer or similar but I just cant see what I'm doing wrong or most probably missing!!!

Please put an end to my misery!!!!

Thanks in advance

Doug