ないことには: a prerequisite for the next judgment
Read a necessary preliminary condition without assuming that satisfying it guarantees the result.
Without the first action, the next step is impossible
実物を見ないことには、買うかどうか決められません。 means that the speaker cannot decide whether to buy something without seeing the actual item first. Seeing it is presented as necessary for making that judgment.
The first negative presents not seeing the item, and 決められない states the inability to decide. Do not cancel the two negatives into a positive; read the unmet condition followed by the resulting impossibility.
Attach to the ない-form
Add ことには to the verb's ない-form, as in 聞かないことには and 読まないことには. 本人の希望を聞かないことには、計画を決められません。 means that the plan cannot be decided on without first asking what the person wants.
The relation is especially clear when details must be checked before a judgment. 詳しい資料を読まないことには、賛否を決められません。 means that the speaker cannot decide whether to support or oppose something without reading the detailed materials, not that the speaker opposes reading them.
Necessary is not sufficient
Even if seeing an item is necessary for a decision, merely seeing it does not guarantee that the decision will follow. The price might also need to be checked. Do not expand one prerequisite stated with ないことには into a guaranteed result.
実物を見ても、決められません。 sets up a different relationship: even seeing the actual item does not make a decision possible. 見ないことには says the decision is impossible without seeing it; 見ても says the impossibility remains even with that condition met.
Does 実物を見ないことには、買うかどうか決められません。 mean that seeing the item will definitely lead to a decision to buy it?
Show the answer and why
No. Seeing the item is necessary for deciding. The decision may remain difficult afterward, and even if a decision is reached, it could be not to buy it.