I've been wondering this too, and I wish there was a definite answer. Until then, here's what I've observed: RIMs in shops are typically either placed at 1 diamond, or gems. The amount of gems varies from person to person.

What I do is I try to account for the wiki's projected worth, and what I assume the demand will be. If the Worth Wiki states that it goes for "a few RIMs," then I generally go for 1 diamond (unless I assume it's low demand). If it's any lower than that, I go for gem pricing. There's no real answer for gem pricing. I go for what feels right.

As someone who primarily sells RIMs and clothing betas, I've found that there's some benefits to selling a handful of stock for gems. You usually clear out inventory faster, and get more visitors (some of which may spend diamonds if something catches their eye.)

Until we get a real answer, that's my advice to you.