Here’s what my point of view has always been: there are certain optimal, or close to optimal, grinds - in terms of EPA - which pay out certain items. Those items are "optimally expendable": if you sell them, but later decide you want more, and go grind for them, you won’t be losing out. On the other hand, any other item is suboptimal to sell.
Let’s say you need 1,000 echoes for something. You sell a vast amount of moon pearls, brass, shrieks, or whatever other low-tier item; you now have your 1,000 echoes. But then, at some undetermined point in the future, you need those low-tier items again! So you have to grind them out. Say you’re using Unfinished Business, those range from 1 to 1.5 EPA or so. You probably won’t need 100k right now, but in the long term, if you happened to end up needing to make most of it back, you’d essentially end up grinding for ~1000 actions (1k echoes at ~1EPA).
Meanwhile, if you took the time to grind out some "optimal" items upfront, using the current best money grinds, you’d get your 1,000 echoes in about 460-ish actions, and never need to grind the low-tier items. So, long-term? Selling the suboptimal items cost you over 500 actions. Investing those into the EPA grind would have instead left you with a surplus of about 1,200 echoes (on top of the initial 1,000).
Of course, this assumes that you’ll actually need those low-tier items later. Reasonable limits can be applied to this: after ending up with something close to… oh, 300k moon pearls? - from going for Poet-Laureate, I’ve sold off the bulk of them over time, simply because while they’re suboptimal I know for a fact I’ll never need anywhere near that many, even if I keep playing for years and years. Unless FGB adds some kind of pearl-hungry action in the future, of course. But I wouldn’t sell them below about 10k; same for other low-tier items. I remember trying to get the Gang of Hoodlums and realising I was out of Rostygold because "Who needs thousands of the thing? I need some money right now, I’ll keep a few hundred just in case". Wouldn’t recommend the experience.
High tier items are even worse: you’re unlikely to end up with hundreds of them, yet you never know when you might need a few dozen for something. And their grinds can often be well below 1EPA, to boot.
Anyway, all that being said, right now the "optimal" items that are safe to sell are the rewards of the silk expedition (Parabola-Linen Scraps and Judgement Eggs) and, very slighly second to them, the tribute rewards from Court of the Wakful eye (Searing Enigmas, Night-Whispers, Favours in High Places, and Primeval Hints if I remember correctly). Anything else will incur a net loss under the assumptions above; though taking a look through the wiki page on grinds might be worth it: the old grinds, while now suboptimal, are often still much better than UB’s ~1EPA, and in a pinch selling e.g. Puzzling Maps might not be too bad of an idea.