Advice on how to go about selling my startup? Team is moving on to other things but believe our startup (an iOS app) is valuable to the right buyer

Like any sale, it's all about building a funnel and identifying qualified buyers. The best way to do this is to ask, who do we share a similar customer with that would like to sell what they do to our users, or that our product helps improve their product.

Companies get bought when there's overlap in the customers, or you fill the white space in their strategic map so they can move faster by buying vs. building themselves.

Once you have the list, then you'll want to create a spreadsheet (hopefully you have 12-20 names) and then identify these 3 people at each company.

CEO - making the final decision.
Corp Dev - it's actually their job to buy you!
Product Manager - internal champion that will decide if you're the right fit for them.

Sometimes these can all be one person (i.e. the CEO) or only 2 people - either way, you'll want to build a relationship with each on quickly by getting an introduction.

You don't want to come off as "hey, we want to sell what we've built" but more, "hey, I think we share a similar customer base and could do something together that would be mutually beneficial". That will get you a meeting.

If you start with 12 companies and do the work, you should be able to get 3 of them competing to purchase your startup and you can use this as leverage.

The key is to get them all to the same point, at the same time (i.e. giving you a term sheet or offer). You don't want one to move to fast, and not let you get the other interest going - or they might pull out of the deal from your lack of decisiveness.

I've written about this in more depth here:
http://maplebutter.com/7-tips-for-getting-acquired/

Hope that helps.

Very unlikely there is any value. The value of a startup is mostly in its people, the founders, etc.

Might be hard to get a bite without at least some of the team members staying on, but worth checking out : http://exitround.com/

It is sad that you must sell a start-up, and your team is moving on, but before you sell the start-up here are few things you need to know first.
There is huge (10x+ valuation difference) between you approaching someone and someone approaching you. This is a very time-consuming process and can take six months easily. Ideally, one Co-founder must pursue this full time. If you are looking to sell, the chances are business has stagnated or you have lost interest and want to move on. You will not get premium valuation if these are the reasons. Also, for SaaS start-ups, the industry standard is often 1.5x to 3x. It should be little more if the buyer sees more value, but it will always be something realistic. Having realistic expectations and understanding the market often saves time. As a founder, you will always feel like you deserve premium, considering the risk you took, the time and money you spent and how you lived with minimum salary. But unfortunately, this will not make any difference in what the buyer is willing to pay. I have seen people selling start-ups and businesses at a loss too, as most times, something is better than nothing. You can sell the whole product with all assets at one go but many buyers will ask you to continue working on it or provide minimum support for X months. There may be a few who want to pay X amount up front and the rest after Y months based on performance, and some just want to do partnerships. Make sure you understand the pros and cons for each option.
Make a list of all potential buyers. It is likely you want to contact companies who are in your vertical. It helped that I already had most such people already on my LinkedIn network. Make sure all P/L documents and finances are up to date. It is worth hiring someone who can take care of the legal work as you can save lot of time later in case something goes wrong and most of the time, it is not worth scrimping on the Rs 20,000 -30,000. At least, have someone read the draft if the buyer is the one taking care of legal work. Escrowing money will always be a problem if you are in India and the buyer in a different country. I am little unsure how it works for big transactions but if you are hiring a broker, they should take care of all this. For big transactions, you obviously want to meet the buyer face to face too.
Besides if you do have any questions give me a call: https://clarity.fm/joy-brotonath