How early is too early to start selling?

Depending on how crappy your team looks. It's hard to say in such a deep league but I wouldn't consider it until the end of May at the earliest... preferrably later than that. When the contending teams are rounding out their squad to make a run and separate themselves, they may be more inclined to overpay.

Well if you feel that way, go for it.

If you think your players are overperforming and you can sell high on them to compete, it's a great idea. But selling high at the end of April to rebuild for next year just doesn't make sense to me... unless your plan from the start was to tank the season.

It's still April.