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When I built my system, I had 6GB (this was a decade ago, that was decent!) then I got 3 more GB used when 8GB became the norm. Now I have 18 GB (for those trying to work the math out, 3x2GB 3x4GB, it's a triple channel first gen i7). The used RAM has worked fine for years.

If it's working and it's compatible, it's ok. Just test it really well if you get a chance before buying. I sold my old ram at least 3x when switching generations (sdr to ddr2, then ddr2 to ddr3 and so on) and it was always perfectly working, sometimes still under warranty. It's not that hard to spot a faulty ram with a good memtest prog.

If it's worth it and Your MB supports it, then I would advise taking up DDR4 RAM. An investment for future upgrades

I think that, you must know what are seller doing with this ram before selling, i mean coin crypt or something

While we were reusing RAM modules at my previous IT job all the time, I wouldn't actually order one online that I could not test before. Maybe if it is the exact same model than the ones that you are already using...
Anyway, DDR3 isn't a good investment and with already 16GB in your system, my guess is that you are getting a better performence if you simply clean up your system a bit. Simply closing your Browser before you start a gaming session can make a big difference.

before buy more RAM you need to know that the actual games (or almost the 99% of them) only use up until 16 GB RAM (and only a few use that 16 RAM)

with that say even with 15156156 million of RAM you will see 0 diference.

Ask for a CPUZ screen to check that the sticks are working correctly and then get them if they work. If there's any issue RAM sticks have lifetime warranty anyway and you can replace them pretty easily.

In my own personal experience, especially in laptops, having multiple sticks that are not identical brings more problem than good.

Used ram is usually good but make sure the seller has good reviews.

16GB of ram isn’t enough for me either but it’s not gaming that causes issues, it the other tasks I do on my system. I got 32GB in my workstation but I haven’t seen more then 18GB get used.

You should of waited and saved your money and bought all the parts you needed excluding the video card as you could do that one down the road as an upgrade. This would of allowed you to get all the ram you needed at once and better parts or discounts on older gen parts as the parts you picked out would of been outdated over the time used saving up.

I would even suggest looking at getting better ram in bigger sizes. Go with 2 x 16GB sticks of something faster then the standard 2400. And just sell off the ram you bought that’s hard to find now.

As for gaming and ram, check this video. And if your planing to get an RTX 2080 TI, then yes go with more then 16GB of ram just to get the full performance out of your gaming.
https://youtu.be/BcJI5H3fQlE

I myself got used ram from ebay so totally go for it, again just make sure the parts fit and read the descriptions carefully, don't be afraid to ask before buying for more specifications too!

ebay by default will back the buyer in any dispute so you're pretty much golden, albeit given the wide variety of arseholes on there can make it a tiresome experience. Depending on where you live another option maybe a proper used tech business rather than a rando off ebay? In the UK we have the glorious Computer eXchange that buy and sell used parts, have a no questions return period and give a warranty on everything they sell. If you're not UK perhaps there's something similar with you?

Edit:

Upon reading your PC's spec, oh my, you chose the wrong platform for a gaming PC. The FX CPUs were never fast, and they're quite outdated now. (I did recently buy a 8350 myself, but that was as an upgrade to a Phenom II; upgrades are limited there.)

So my thought is that you shouldn't invest more money into that PC, and look into moving into something more modern, like Ryzen.