Your best bet is to take some wide angle (not fisheye) pictures, and send them to online agents and get them to sell the flat for you. They'll post ads for you on squarefoot, gohome, propgo, etc for free, and then if they're able to sell your flat, you pay them 1% commission (you can ask for a discount, but that's like eating out in the US and screwing the waitress on the tips. People do it but it's really frowned upon). You will reach a lot more prospect buyers that way then by posting ads in newspapers.

Leave the key with the building management guys or give out keys to your local agents (Centaline, Midlands, etc. Yeah they're assholes, but it's easier for other agents to find them to borrow the keys) if your building doesn't have a management office, so other agents can borrow it. Generally, the easier agents can get access to your flat, the easier they can sell it.

Or you can sign a sole agency with a reputable agency for like 3 to 6 months, they'll send out EDMs to other agencies for you. And instead of getting hundreds of phone calls from different agencies, they'll call your sole agent instead. You could then call them up once every week or two to ask how many enquiries your flat has been getting, etc. The problem with sole agency is that the agent will most likely retain their commission, that makes it less attractive to other agents (they would try to push the flats that pay them commission).

With regards to bank valuations, different banks give out different valuations. It doesn't necessarily depend on the bank, they will give out higher bank valuations to flats in different areas/districts and it changes all the time. With that said, Wing Lung Bank tend to give out higher valuations at a lower interest rate.