Yeah that's an EVGA 450 BT PSU on the 2nd build, not a Thermaltake. Amazon has it averaging 4.5/5 stars on 3748 customer ratings, but jonnyGURU gives it a paltry 6.5/10.
https://www.jonnyguru.com/blog/2018/11/26/evga-450bt-450w-power-supply/6/
I don't know, there's issues with both builds. You have to wonder how hard the 1080 has been pushed on the used build if the seller sees fit to bias the Heaven score. I also agree on slim cases having poor space/ventilation, and brick PSUs being lackluster. I also think given those reasons it's a bit overpriced.
That said, with the 2nd option you'd be buying into quite a bit of rebuilding to upgrade an old platform, which involves replacing CPU, MB, and RAM to the new standard, PLUS the GPU is already a bit weak in comparison for some of today's games.
I have to say I see red flags here on both. How much time did you put into searching for these options? Seems there has to be better ones out there. Keep in mind too we are heading into Black Friday and the holiday season in general soon, and lots of people will no doubt buy new PCs and sell their old ones. So there will likely be more and possibly better options soon.
This is an important decision and one you will likely live with for 3 years or more, so take your time making it and weigh options carefully. I also advise asking sellers questions, you can often get a feel for their integrity that way. I've used eBay a fair bit, it can be a great site to buy from if you know what to avoid and how to get the most out of it.