First of all, you need to find out whether or not the weapon is popular/meta. I'm not entirely sure how you can figure that out easily.
I normally pay a little bit of attention to how often people bring said weapon to missions, clan chat and community posts in social media (the longer you play the game, the easier you'll figure out a weapon's popularity on your own). I think you can divide weapon popularity into three groups:
1- Collective delusion. This weapon doesn't exist. (not worth trying to sell a riven for it)
2- Known around the block. (might be worth trying to sell a riven for it)
3- Everyone and their mother loves this weapon. (definitely sell a riven for it)
Second, you need to figure out which stats are the most desirable for said weapon. You can figure that out on your own by just checking that weapon's stats and/or how you mod it (ignore the latter if you're the type that likes to swim against the tide).
Some stats are universally desirable for every weapon, like +Damage and +Multishot (maybe +Attack speed too, but don't quote me on that).
If said weapon benefits more from crit stats, then Crit Chance and Crit Damage will boost your riven's value much further than any other stat, but that's not to say that all other stats are bad and irrelevant. They each have their own value, but other stats' value wouldn't be as high as Crit Chance and Crit Damage (not even universal stats), in this case.
A negative stat that doesn't hurt the weapon will boost any rivens' value, especially the most sought after ones.
Third, you gotta decide whether or not you want to gamble roll the riven.
It's really up to you, your kuva stock and your time.
Unrolled rivens are valuable for weapon popularity 3-, but that won't boost your price by crazy amounts either. At best, you get an extra 50pl~100pl for that on a popular weapon (3-), nothing~30pl on a not-so-popular weapon (2-), and nothing on bottom tier (1-). You only pay extra for unrolled rivens if you're really trying to save those 10k~15k kuva from the first few rolls.
Godroll rivens (the "perfect" roll) don't devalue with number of rolls, nor are boosted by being unrolled. Anyone who tries to devalue your godroll riven because it's been rolled, is trying to scam you or don't know how rivens work. It doesn't matter if it's rolled 1 or 100 times. A godroll riven will not be rolled anymore, because it's already too good, perfect.
The number of rolls affects your riven value THE LEAST. The actual stats and weapon are the biggest players in gauging your riven's worth.
Last, but not least, set your own standards for rivens over time.
How forgettable a weapon must be for you to insta-transmute a riven or sell for 5pl?
How many rolls you wanna put into a riven before trying to sell it (if any)?
Minimum price for OK-ish roll for popularity 2- weapon?
Minimum price for OK-ish roll for popularity 3- weapon?
Maximum price for great roll for popularity 2- weapon?
Maximum price for great roll for popularity 3- weapon?
Among other things you have to find out for yourself.
Don't be draw too much by the 5000pl end of riven sales. That is a [lucky] minority.
There are a lot of people selling OK-ish rivens for 50pl~200pl.
When a new popular weapon is released, it's common to see people pay unreasonable amounts of plat for the trashiest of rolls, but that only lasts for a few days. Try to take advantage of that period. This applies more to new weapons than new variants, as new variants already have a S#&$load of rivens just lying dormant in many people's accounts.
Disposition has no direct impact on riven value, but it might affect popularity.
MR requirement has no impact on riven value.