Aside from that not being eBay's policy, although you might get away with it, it would not be wise to do that unless you are willing to pay $400. What if someone really, really wants it and bid $380? Your bid of $400 would trump that, probably at $382. Your friend might not be very happy at accepting $250 from you when he could have had $380. How well do you know your friend? Are you so sure it will work like he said? It might but it just as easily might not and if eBay gets a whiff of what is going on, you both will be banned from using eBay in the future. I suggest you bid the highest amount you are willing to pay for it and if you win, pay it. If you don't, then he will get the amount that someone else was willing to pay - higher than what you were willing to pay so, the other guy deserved it more.
eBay Buyer & Seller for well over a decade.