FWIW- If the customer makes first "offer", he is then at the disadvantage in the deal. Having the dealer start off gives you the opportunity to see if his first price is best, based on your research or- shows you he starts high.
Example- I "offer" $100.00 for the "purchase" The dealer smiles because his starting point would have been $90.00 with room to go down. You have already lost- the dealer might say then, that $100.00 is too low and get the buyer to go higher.
If the dealer goes first then you know exactly where he stands. If compared to your research he is high- than negotiate down. If he is giving best price then no need to negotiate further.
Using the "build your own" is a great tool to know where the MSRP is. If you wanted to play hard ball, Knock say- 15 percent off and have that be your counter offer. BUT playing hardball might end the negotiations then and there.