Sight unseen
Definition 1 of 1
Pro Tip 1/3Trust Requires
The phrase often implies a high level of trust in the person offering the item. 
He rented the apartment sight unseen because he trusted the landlord's reputation.
Pro Tip 2/3Risk Involved
Using 'sight unseen' suggests there are risks since the item wasn't inspected. 
They purchased the horse sight unseen, hoping it was as healthy as described.
Pro Tip 3/3Online Transactions
The term is common when buying items online where physical inspection isn't possible. 
I got my wedding dress sight unseen from an online boutique.