Lots of questions to the original question. I guess you won't get out anything much unless you reveal the domain names
I think you have 2 types of domains:
Brandable domain that can be used for a broader niche or the domain name itself can be used as a name of a brand or service. For such types, the metrics don't matter, but at the same time, the value is what the buyer see's it to be.
For ex: if someone is dealing ACME Widgets, buying acmewidgets.com or bestwidgets.com might be of value. If they think they can spend $100 so be it. But if your asking price is $500, then forget it. Unless its a super brandable domain name.
Generic domains: These are the ones more suited for the masses, hence a wider audience. Metrics matter here. Especially in this age of PBN's. If you have domains that are of decent metrics and if they can be used either a money site or a PBN, then its good.
The value again here depends on the market. If one could get a similar domain with same or better metrics then it gets equally difficult for you to sell.
You need to do some research into the domains you bought to find a unique selling point.
Is it a generic domain? Look if it has good metrics. A DA20+/TF15+ should be easy to sell, but it comes down to how much you want to sell it for.
For your costs, remember that you will have yearly registration costs that can become an overhead to your expense already. Flipping would be a good idea if you manage to get the domain at a low cost and sell it. Also remember that the longer you hold on to a domain, the metrics might begin to deteriorate as the broken links get fixed and so on.
I am mostly active in the finance niche and it is absurd to see how some ppl expect to sell a domain for $500+ and market it as saying it is a top money making niche. There are also domains that are priced at $2000, with crap metrics. The holder of the domain just keeps holding them for years at a stretch, with little success.
Bottomline: evaluate your domain properly, figure out where it can be used best. Research the niche, metrics and then give a fair value.
In today's age of expireddomains.net, domcop, flippa and the domain registrar's own marketplace, it is not that difficult to find a decent domain with good metrics.
Unless you have a domain called example.com, best to get the expectations right and try to flip them as soon as you can do you don't end up holding the bag at the end.