Don't let anyone sell you a bike just because it's right there on the floor on sale. You might like that bike but might need a slightly smaller size of it, for example. They could order that smaller frame for you. Getting the wrong size bike frame to begin with (perhaps because it's on sale) and then trying to "make" it fit you by switching/tweaking components is not the best approach. Better to get the right size frame and then have the ability to tweak the components to fine tune the fit. You want to be comfortable so that you'll want to ride your bike as much as possible and generally fall in love with riding.
On...the...other...hand...

Buying the bike on sale that mostly fits can allow you take advantage of your initial enthusiasm, good weather, and whatever bubble of motivation is working in your mind right now. It's what I did when I bought my first road bike. There are degrees of comfort, which you can trade off for riding now, or riding later; for spending X that you have now and riding now, or for saving enough for X so you can only ride later (sometimes much later).

We have friends with whom we kayak and camp a lot. They spend so much time making decisions about what gear to buy that they still have to rent kayaks when we go, while I'm happily paddling around in the one I bought years ago on a whim, and I can go any time, anywhere.

I grew up riding any bike that I could pedal, so the idea of not having "perfect" fit before I start riding is not so foreign. Plus, how would I know about everything I want to fit right if I don't put some miles in? You could spend big bucks on a bike only to find out many miles later that you wish you had bought something else.

I'm suggesting that just "good enough" is sometimes good enough. We don't always have to strive for perfection, especially when we're trying something new.

Karen