Quick answer
Once the offer is accepted by you or the seller, the listing will be considered purchased by you.
You'll get an email notification as well as an Unpaid Order notification in the upper right-hand corner of Reverb. You'll then be able to check out and pay via the Purchases Page in the Buying section of your Reverb account.
Abandoning orders after your offer is accepted creates extremely frustrating experiences for sellers, so Reverb frowns upon it strongly.
Buyers with multiple abandoned orders are subject to negative feedback or account suspension. Please only make an offer on an item if you intend on following through with your purchase.
Further reading:
What happens if you don't pay for your accepted offer?
How do offers work on Reverb?
Can an offer be cancelled?
How long do I have to pay for my Reverb purchase?
Why was my credit card declined at checkout?