Really??? I mean, I sell slime balls for 38 rupees each, and dozens of other people sell 'em for reasonable prices too. I think you may be trying too hard with this auction thing, lol.
You might be able to get 450r for an entire stack, but since 1.0.0 update slimes seem to spawn alot more, which reduce their value considerably, and as dark_liz mentions, players sell them dirt cheap these days.