What a great problem to have, and an awesome question for Clarity Experts.

My brief take is as follows:

1. I agree that direct relevancy doesn't exist between their information needs and your tax returns
2. They will definitely be looking at the value they'll leverage from ownership - not its net present value to you, and will base their internal value (ie what they'd pay for it) on that.
3. The P&L will be more useful to them - but may be superfluous info (they've been paying you for leads that they know are at a markup - so there is obviously margin to be gained).

That said - you can do both. Not giving them what they ask for might create conflict or impasse.

At the end of the day, it will come down to you both agreeing on a value - one that feels good for you - and works within their economic situation.

Bear in mind a few things - selling a business isn't a decision in a vacuum - you're in a market, and there are likely alternatives. Also, as you attempt to determine an empirical value - obviously maximizing the price in your favor, you should not lose sight of the value to you of any offer - ie, if you currently net $100K, and they offered $400K would that be a better value that continuing the business for 4 years to generate the same cash.

Empirical value is okay for setting a starting point - but in the end - you're going to have to negotiate to a point of agreement.

You can provide them what they want - and present your take on the value to them, but ultimately THEY know what those leads are worth, and may have plans beyond what is obvious to you - ie, even your rosy empirical value may fall short of what they'd be willing to pay.

So, if I'm in your chair - I'm thinking long and hard about what I want out of it - I'm presenting a starting point that feels reasonable, and I'm working to land as close to that as I can.