Escrow.com is a good option on high-dollar transactions where you don't know/trust each other 100%.
You guys would have to work out who pays the fee, or if you split it. Even if you have to pay it, it should feel worth it for the peace of mind.
As long as you're willing to pay the fee, I don't see why a seller would have a problem with it.
With or without it, it's perfectly acceptable for the seller to require payment before releasing the domain. That's pretty much how it works even on small $5 PayPal domain sales.
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