Question

Is the problem of something merely appearing like interest a problem vis-à-vis other people, who may make a mistake and think there is interest here, or is there in rabbinically prohibited interest also a real problem of interest—meaning that on the basis of the same principle that makes lending with interest prohibited by the Torah, the rabbis found additional cases to prohibit because of that same problem? And if only the first side I wrote is correct, so that everything depends on what people will think, then why in a case of rabbinic interest such as price-fixing in advance, when this is done in a situation where he already has the produce at home—so that since it became more expensive while in the buyer’s possession this is not interest—does the fact that he has it at home remove the real problem of interest, and not what people will think?