When it comes to privately buying or selling domain names, I would say that I am a stickler for contracts. Even when I do private deals with people I know and trust, I almost always use a purchase agreement. In the event something unexpected happens in the short term or over a longer period of time, I want to make sure everything is outlined and agreed to in writing.
I understand why a purchase agreement can be burdensome. It might be silly to agree to buy a name for $50 on NamePros and ask the seller to sign a 4 page contract. Purchase agreements can also be expensive to have drafted for a business, and a boilerplate agreement may not suffice.
That said, I would be interested in knowing other people’s habits when it comes to purchase agreements. Feel free to share thoughts in addition to voting below.