I tend towards another course.
The demand for and asking price for used GPU are falling faster than the return to MSRP for new ones. Although it remains to be seen, there is talk that we will see new product, readily available, and below MSRP in May and beyond. Many buyers are taking a wait and see attitude.
The 9600K in your system is just old enough that all the cheap new stock of upgrades like the 97/9900K are gone, so things are going to the used market on that as well. The newest line of 12th gen Intel (aside from the pressure from AMD) makes a whole lot of sense from a buy new rather than used standpoint based on its price and performance. In particular the 12100 and 12400 are strong performers in the budget category.
There is a balance of really nice parts in your PC. Mismatched RAM isn't going to help. HDD is a nice value add for the whole PC, but is among items that would likely not sell, or for much, on it's own. The market here around Atlanta is very competitive. I would imagine that if you tried to sell used, here, it would be ~$700 on the high end, whole and probably a bit less to sell quickly. I do think the GPU is the most valuable part but would not expect it to move as quickly or for the premium it would have drawn a month or so back. I would make sure everything is clean, maximize the lighting, perhaps even add a cheapo strip to two to really bring that out. Take some good pics, ask a bit higher than you are aiming for and be ready to adjust downward based on demand.
Parting this would result in the best price on certain aspects of it but will leave you with a pile of things others don't want IMO. I feel a bit like making 'combo' would both maximize your return, as well as not leaving you with part of the build.
For instance, mobo, one set of the matched RAM, CPU, OS drive (maybe even the cooler) as a combo.
Case with the fans and so forth as another combo.
GPU will remain the most valuable item, but keep in mind that demand and pricing for that is going to fall quicker than retail towards MSRP on new units.