Molex to pci-e is not safe and neither is that PSU.
That PSU is absolute trash. It can only deliver 9a on the 12v rail(thats 108w) so it is not even remotely close to a 700w PSU and not nearly enough to power a r9 280. A good 700w PSU will be able to deliver around 53a on the 12v rail(s). That has to be the absolute worst "700w" PSU i have ever seen.
The PSU is a very important component and a very bad place to cheap out.
x1000 on the above.
Also, in general, molex-to-PCIe adapters are NOT safe, and are likely just begging for a component failure, if not a fire.
Click on the first link in my signature. It is a MUST-READ when it comes to getting a good PSU.
It won't be cheap, and, unfortunately, the pandemic has made the prices even higher. That said, a PSU is a safety device that protects you and your PC components. A good one won't fail easily, and if it does, it will kill itself and keep the rest of your PC safe. A good PSU will, figuratively speaking, throw itself on the grenade to save you and your PC.
A low-quality one will not deliver clean/steady power, thus shortening the life of your parts. A low-quality one will also, when it fails, run the risk of killing other parts of your PC, or possibly catching fire.
If you absolutely MUST keep the price down, the second link in my signature offers a list of what jonnyguru would consider acceptable for a PSU. Pay heed to what he says about the downsides of using the cheaper ones, but at least they are cheaper but of decent quality, as opposed to cheaper but dangerous.
Final note: 80 Plus ratings (Bronze, Silver, Gold, etc) are only EFFICIENCY ratings. They say nothing about quality. The efficiency is, well, how much of the power that goes into the PSU comes out as usable electricity, and how much is waste heat. Higher efficiency = less waste. HOWEVER, a bad quality PSU
can be efficient, at least until it craps out.