Granted, the going price on the platform you use for selling is the price you will look at. People selling on eBay also have different selling prices than what I'm used to, and then there are price differences between regions in the world. Sets that are rare and expensive in the USA might be more prevalent in Europe, and vice versa.
For me, in The Netherlands, my #1 platform to sell on is Bricklink, knowing fully well that on average the prices there are lower than on other platforms.
Regarding Kylo Ren's shuttle (75256) still selling for ~ original retail price on Bricklink (EU) I think is caused by the sequel trilogy generally not being very appreciated by Star Wars fans. I feel the same way and am not at all interested in sets from the sequel trilogy. I even sold my Resistance Bomber as a 2nd hand built set (with a small profit even), but I'll keep my 75256 set because it's still a great looking set and well designed. Also keeping BB-8 because it looks nice next to my R2-D2 UCS set.