Just add a lot of utility for the pixels token to promote buying pixels. There aren't much perks of buying pixels, so people are just selling them for profit.
Add a coin bank where you can add lists (3 free) then you can add new lists by paying pixels.
Expand bookmarks by paying pixels.
More cosmetics option using pixels.
Pixel lottery (just make sure the amount of pixels being collected by the team is greater than the rewards).
There's a lot to list, but my only request is just make loads of utility for pixels token and you wouldn't really have to worry about sustainability.