How much do aged sites go for?

Hey,

if you made a blog, put 5 to 10 quality posts on it, then left it for a year, how much would it sell for on bhw or flippee?

It depends of the niche.
If it isn`t indexed by Google and the traffic is 0,the value of such blog might be also 0.

for 0 to whatever ......

What anything sells for is the value that the buyer see's in it.
For buyer 1, if the domain has good metrics and they are shopping for PBN's then your domain might be what they want
For buyer 2, they are looking for an EMD, which assuming your domain/website is, there is value right there

Just because a site is aged or a buyer is demand xxx,xxx for a domain doesn't mean it will sell for that. If you actually spend time looking at some expired domains and use the wayback machine, you will see some of the domains listed on SEDO or parked and demanding $xxx. When in reality the domains were shi!t.

So, focus on building some good back links, make sure that backlink profile is neat and list on BHW, given the # of PBN sellers here, it would sell quickly for a decent amount of money.

What if it's indexed but 0 traffic, niche is a popular one

It should be popular one...indexed, ranked and unique content will let it to stay in ranking.

You can value a website/blog depends on the revenue from it. If you are not monetizing or not getting traffic means you can consider it has zero value. If you are lucky, you may get few bucks not more than that.