Uhh... that is completely normal.

It's also sometimes called the earnest money agreement because it is at this time that the buyer usually puts down earnest money of approx. $1,000 or 1% of the sale price to be used towards the purchase of the home. Generally, the purchase agreement contains contractual terms regarding who gets what appliances, what the price of the property will be, any items that may need fixing, how quickly the transaction should close (usually 45 days), etc. Usually the agreement is done on a standard pre-printed document with certain items added or removed based on the mutual bargaining of both buyer and seller (generally through intermediaries, like realtors or lawyers).
The buyer almost certainly cannot finalize their financing without such an agreement. Also, it would be at this point that the buyer needs to spend some money in getting his financing, like purchasing an appraisal and home inspection of the property, etc. He needs your signature stating that you're not going to d!ck him and decide not to sell or try to sell the home to someone else.
I'm assuming you're not using a realtor (because they're not necessary right?
). If you don't know about these things, I suggest you hire a real estate attorney...