In general, if you are looking for a system to play games with, building your own is the best option, if you have the money, buying from an SI is the next best (they basically do the same as a custom build, it's just that they're doing it instead of you), after that, you can get a gaming focused, used, prebuilt if you like (like an Alienware or something similarly gaming focused) and do some upgrades (GPU, disk, more RAM, etc) as appropriate, or buy a prebuilt office PC and add a good GPU.

The custom built after market is a nightmare of both good and bad deals; for someone who isn't completely versed on hardware, I would say you either need to bring a friend who is, or just avoid it entirely.

With all used/refurbished systems, always set aside some money for a new primary drive or SSD, since the one that's included is probably fairly worn out, and it's not unusual to have it fail within a few years of getting the system.

The only new PC's I would say you should consider, are from SI's. Where it's basically a custom/self built PC, but built by someone else. Only if you have the money and only if you can't do it yourself for any reason. If you have any technical know how at all and can take an hour to look up PC building guides, then spend a few hours on pcpartpicker or similar to spec the system.... Then just do it yourself. I won't fault anyone for using an SI, if they simply don't have the time to learn and do it themselves, or if they have a mobility issue... (among many other reasons). Buying a new prebuilt from a big name should probably be avoided where possible (names like Dell, Lenovo, Microsoft, HP, Asus, Acer, etc). Used are fine as long as you can get a deal and the system is part of the business line.... For gaming, not a lot of business PCs are good for gaming. Some can be upgraded to be decent at it (usually by adding a consumer graphics card).

The difficult one is laptops. If you want a mobile system with graphics enough to play modern games at even modest settings, you're going to have a lot of difficulty finding something in the used/refurb segments... Mainly because GPUs have such a significant performance difference between each generation. Any modern generation GPU in a laptop will command a very high price, and it goes downhill fast, especially considering that mobile GPUs are fairly poor for performance, even compared to the same generation of the same series of desktop card.

In those cases, I'd generally recommend a business system with a thunderbolt GPU dock, and just slap in a desktop GPU. It's not as mobile, but you're going to save a lot of time and heartache trying to find a good system that fits both your performance requirements and your wallet. An external GPU dock gives you the flexibility of using less expensive desktop cards with more power, and upgrading that card whenever it suits you.