We suppose that two items are similar if they were* bought together by
You only 'suppose' if you have evidence to support a supposition. So when put together, these bolded statements make sense. 'If' is the modifier here. Your supposition only exists because of the word 'if.'
*You use 'were' in more affirmative statements. It it like subjunctive, but not quite - since English has no subjunctive, truly. It only has it in some cases. It is suitable for proving theory, since it implies you know that if products are bought together, they [then/therefore] are similar.
- You use 'have been' in more present situations. If things are being
bought right now, right here, in this moment, then it is
appropriate to use 'have been.' It's a more hypothetical case. It
means you only suppose your supposition if the following (buying
products) has occurred. If it hasn't, then you don't suppose
anything. You continue on supposing that they're dissimilar, until
they are bought together.