I need advice

I just recently got into wanting to build pc's and flip them, is there any tips anyone could give me? Like where to snag cheap parts and what offers are good (UK btw), how much computers would sell for, etc stuff like that.

I came up with a pc build using used items from FB marketplace, and new items and I just wanted to know how much would this go for, or is it a bad setup that wouldn't sell, do I need to redo everything?