You have received some excellent answers.
A key aspect to understanding the appraiser's role is that price does not equal value. As you know, you can buy things under predom market, or over. As an investor, you are always trying to buy under the market (Surely you realize some of the prices you pay are below market ), and most investors try to sell a bit over the market if they can. Whatever the situation, just because a buyer and seller had a meeting of the minds on price, our concern is to find the Market value of the property, which can be the same as sales price, above or below. In addition, concessions or special financing can affect price and an appraiser would adjust for that.
Imo, a seller meeting an appraiser at a property does nothing to change the value.I personally become annoyed at very "pushy" sellers, and no, handing an appraiser comps you think they should use does not change things either.
Best of luck in your ventures, and in this market, while there are still REO and short sales out there competing with your resales, you would be best off considering this will happen from time to time and be willing to re negotiate your contracts if it does happen.
As you see, buyers are willing to pay quite a bit more when it is not "their" money, I would imagine you buy your project houses all cash.