Sorry.... I have always known $900 minus $400 to equal $500. Don't know what I was thinking.
And, the simple math I referenced would tell you......if he sells them for retail ($900), he'll make a $500 profit. You DO know what he pays for them. I told you (and he didn't dispute it).
If I were a dealer......I wouldn't be happy about it (someone being able to buy and sell bows cheaper and for more profit than I). But, you're obviously OK with it. No problems on my end.