I have no experience at all, even selling HDB, but managed to do it after asking around.
I first surveyed the prices and decided on what I want to sell for (give room for negotiation), and then put up an ad to gauge the responses. Once I have some calls and ascertained that I am about to close a deal, I called the law firm that handled my RT purchase for advice. I have them handle my sale as well.
Basically, you need an offer to purchase form (the law firm can help you draft if your are letting them handle your sale), so that you can receive the 1% when you have met and agreed the price with the buyer.
You have to advise the buyer to get a solicitor to manage his/her purchase, whom will also help them to exercise the option within, typically, 2 weeks, with a payment of another 4%.
The rest the law firms of both parties will handle.
Personally, I feel that the agent is only relevant, if you have problem marketing your unit (RT?? no prob), or secure the price you want. If you think that you can do both by talking to a few people on the phone, negotiating, that is 2% savings !!
Well, it is up to you, some may still think it's too much trouble.