If I sell decorations and lose more flowers than my current flower balance, will this create a negative..

First of all, the value shown is the amount of flowers you need to reach the next level. To advance, you want that value to reach 0. Once you do, it resets and shows how much more flowers you need for the following level.

When you sell, that number goes up, meaning you will need more flowers than you need currently. To answer your question, you will be in greater deficit than you started, and there is no cap to your deficit.

Avoid selling. But if you must, be aware that you'll only get half the coins back that you paid for it.

You get a negative balance. The more things you sell and lose flowers the more items you're going to have to get flowers from to get your balance back.