escrow.com quotes 26k USD on a $3m transaction, about 1%. I can understand the buyer wants to save 13k cost, but overall it's a small part. Escrow shows the seriousness, you will know $3m has been transferred. If the buyer rejects the product (source code) they don't automatically get their money back. In case of escrow.com they want to see third-party arbitration first (buyer and seller have to come to an agreement). After a month though they'd get their money back.

I'd use escrow. And go further and across the world, sit next to them with a laptop, print out every file or rent an office next to theirs to avoid sending them the full source code. And I'd expect them to pay all the expenses if the deal doesn't happen. If they chicken out of 10k cost (again 1% of the total transaction) I'd say they're not serious.

Third option is to demand due diligence by a third-party.