The reason is that many of us complaining don't like the idea that someone can simply cut the line we've been standing in just because they have money (or are willing to use that money). We feel it's a slimey move to basically sell a type of end game gear that should be worked for.
The extreme strawman is selling a maxed out character (legacy, valor, level, gear, etc.) from the Cartel Market. The problem is that this is a "tip of the iceberg, a possible start to a very bad trend." Many of us who are complaining feel they've already crossed the line.
I for one, won't mind if the line stopped HERE, but BW already crossed it. What's to stop them from moving it a little further... and then just a little further... until it gets to a point where people are buying endgame gear?
I'm going to reiterate: the selling of ship items that are top-tier doesn't bother me. What bothers me is that I feel like BW is sort of testing the water... which makes me feel like they may go down the dark path.