You'd be surprised how many people are willing to buy trash rivens. (I'm guessing some people enjoy buying rivens, rerolling them until they're usable, then reselling them, but I'm satisfied being the guy that makes a quick buck)
IMO it's always worth putting up riven you aren't going to use on a trading site. Personally I use
https://www.wftrader.com/ - you have to create an account (they don't send out any emails or anything), but the layout is pretty easy to use. Alternatively, you could post screenshots of the stats on
https://www.reddit.com/r/wartrade/, but that place seems less active.
Some people seem to spend a lot of kuva on rerolling, but personally if I don't like the weapon I just put them up for sale for 50ish plat (unless it's a really popular weapon, especially a shotgun or bow, then I can put them up for 300ish plat and know that people will buy them quickly since most folks ask for 500+plat for even some badly rolled mods for those weapons) rather than spending a long time farming kuva and rolling the same weapon for maybe a 25 plat increase.