I've practiced in real estate, and my boyfriend owns an agency, but this is all South Africa unfortunately.

Is it not possible for the agent to join another agency, even if only for a few months before her retirement?
Selling a house on your own is a bit tricky, and if you do not use the assistance of an agent, I would recommend at very least to consult an attorney. They can draft sales agreements, and double check any small print that a potential purchaser may add.
Using an estage agent is very important, especially if you have never sold a property on your own before; there are so many small clauses, and so many crookes out there that could make your life a nightmare.

I hope your property sells quickly (I'm sure it's a great peace of mind once it's sold!) and things go smoothly.