https://www.escrow.com/
Don't sweat the amount - it's no biggie for them - I've personally done numerous deals via escrow.com from hundreds to 6-figures with zero problems.
1. Ask the seller to establish a free account at escrow.com; you'll need their account name for opening the escrow.
2. Agree on terms - obviously you've done most of that already, but you'll need to determine who pays the escrow fee - you, seller, or split before-hand.
3. Open the transaction at escrow.com and follow the simple instructions.
As with any deal, make sure you have researched the seller, at least to some extent - hopefully you have/will talk *voice* on a real phone (*not* instant messager, netphone etc), research the domain (check archive.org, check past history, TM issues, etc), etc.
KRL's offer of help is definitely worth considering, since a law firm can often also perform a similar function - may be more cost effective ... compare and see what works best for your needs *and* that of the seller - escrow.com is well known, trusted, and fast - the seller could have their money Thursday, if the escrow is started and domain transferred today.
Ron