If you use Gerber Brand onesies, you can use the word "onesie" because that is what you are selling. Gerber owns the rights to the term "onesie" and you can't use that term unless you are using their product.
If you are selling Carter's or Osh Kosh or Gymboree or any other brand, you have to call them bodysuits or one-piece or baby shirts. Anything else but onesie.
I use the term onesie in my listings, titles and tags because I use Gerber Brand and only Gerber Brand.