$11k is a good offer, but I'd try and find out what the buyer has in mind for this domain. This can be tricky to do, but you can start to guage if this is an end-user buyer by how badly they want the domain (via the way they handle the negotiation on their end).
Find out what the breaking point is for them (ie. the point where they just can't go any higher on the price). How they get there will tell you a lot, and once there you'll have a decision to make on your own. You'll never know for certain if you're leaving money on the table by selling, but if you can get a price you're happy with, take it and feel good about this one. If you need to hold it, your name is of great quality and other strong offers are likely down the road.
Finally, I too recommend Moniker.com's escrow services. Unlike escrow.com, they handle the transaction on both ends. They will accept the money from the buyer and then they will also have you transfer the domain to their (Moniker's) control. Both of those in hand, they can then exchange the goods (domain to buyer and money to you) and guarantee a successful and safe transaction. Their fees are a tad higher than escrow.com, but well worth it in my opinion.
Good luck with this one and congrats to you thus far!
zesty
