Just some random thoughts on this. Before I read it, I had thought of the unique code idea to link the buy to the seller.
- You could have a screen with a text entry field set to fill a user specific column. This entry field will be used to enter a code.
- Then use that user specific column to create a multiple relation to the sellers list. You could then display an inline list of the matching seller that matches that code.
- Alternately, you could make the relation a single relation, then use a lookup to pull back the Seller name.
- Once a match is found, you could display a form button and have the buyer click on the form to submit their email and the seller code to a sheet.
- Finally, you could build a relation and lookup in the sellers sheet or products sheet to link to the form response sheet and pull back an array list of all buyers emails associated with that seller.
- Then you can use the list of emails to filter by signed in user.
This should allow for a single buyer to be linked to multiple sellers.