Buyer's etiquette

Hi there, it's my first time buying bike and also my first time buying second hand motor vehicle here in Australia, and I am wondering whether I am a cunt in this situation.

This Wednesday, I inspected a Honda 125 for like $1300, and I basically promised to the seller in writing (though only by Facebook text informally without legal binding) that I would purchase his bike as soon as he gets the pink slip done as soon as this week. However, he failed to do it this week due to his personal unknown reason, but I forgiven him said it's okay that I can wait till Monday.

Today, I inspected another bike of same model for $1290, and I told the seller that I really wanted this one, but I had another deal with another seller, he offered me $1100 only for today if I change my mind.

Both bike passed ppsr, however, the $1300 had a broken rear light (brake light is working), that the seller promised he will fix it to get the pink slip, and also has more rust due to stored near the beach.

Is it rude for me to reject the former deal to accept today's $1100 deal, or it's considered very rude here in Australia to do so? Also what excuse should I use?

Any response is welcomed, thanks.