@Jason Quick gave you the quick rundown on valuation. Market Approach or Sales comparison approach, income approach and cost approach. Honestly, as an appraiser, valuation is not a science. You don't gather data, throw it into a computer or formula and it spits out a value. Valuation is an art based on principles. Valuation comes down to interpretation, comp selection (even the income approach to value uses market data from "comps") and validity of the techniques and data used.
Valuation is more important for short term flip style deals when the exit strategy will take place in a short period of time. Market conditions change, and so valuation becomes less important on hold properties. Honestly, FMV is an over rated piece of investing for hold properties. The income stream and the financing are the important factors.
Our Portland market is crazy with "investors" paying cap rates in the 4-5% range (or less). Or Gross Rent Multiplies of nearly 200 or above (yes, $3,000 in gross monthly rate equates to $600,000 in value for some neighborhoods). Why compete with that? Run numbers that make you comfortable. Negotiate with a seller based on what they need, not want. Structure the deal to meet what they need, while meeting your goals as an investor at the same time. Some of our best deals are when we paid at or above "market value" for the property. These scenarios worked well because of the way we structured the financing.
Financing and Real Estate are typically combined, but should be looked at and analyzed independently. A "great buy" can be ruined by bad financing. A "bad deal" can be saved with the right financing. Don't beat your head against a wall battling a seller looking to get offers you are not willing to make. Instead, respectfully decline, let them know why the deal does not meet your goals and leave on good terms.
Listening to and understanding the seller's needs are much more important than what the property is worth (in a hold property anyway). If the seller is fixed on price, work with them instead on creating a finance structure that allows to meet price and still create cash flow.
A mentor of mine put it very simply today. "As investors we are looking for a receptive audience in the right location. An audience that happens to own RE in a well located part of town". Let your ears, knowledge of tools and the market do the rest.
If valuation is a big concern (which it should be in the right scenario) I suggest getting intimate with your investment area. Know the players in the market, know the comps, know the cap rates, know the GRMs, know when a property hits the market, goes pending and closes. Keep an eye on outliers, talk to sellers on a consistent basis and become your own valuation machine. You'll learn more from looking at 50 homes in a certain price range than any appraiser can tell you about a specific market or property. Spend time getting to know your arena and you'll become a better "appraiser" than the licensed "appraisers".