I rented twice before buying, so here is what "looks rented" actually means, because it is not snobbery and it is not mysterious.
It is fit, and specifically it is that a rental is sized rather than fitted. You get the nearest size in stock, minor alterations at best, and the fabric is chosen to survive dry cleaning many times, which tends to mean it hangs slightly stiffly and photographs a little flat. In a photo the giveaways are the shoulders sitting wide and the jacket collar standing away from the neck, and once you have noticed that you cannot stop noticing it.
Where renting genuinely wins:
- Black tie, or anything you will wear once ever
- A dress code you would not otherwise own anything for
- Very short notice
- Your size is changing
For an ordinary suit you will need again in eight months, it is the worse deal both financially and in how you look. Two rentals is often most of the price of a suit you keep.
One thing I would say for renting: if you go that way, book the fitting early and be willing to say the shoulders are wrong and ask for a different one. The staff expect it. Most bad rental fits are people being polite in the shop.