Treating heter mechirah produce as having kedushat shevi'it
Question
I don't believe in the halakhic basis behind היתר מכירה. However, I live with roommates who do not see it as a problem, and our food is shared. Invariably, I end up eating היתר מכירה. Since I do not believe in its halkhic basis, and, however, do end up eating it since it's incredibly difficult otherwise, should I consider all the היתר מכירה fruits and vegetables to have קדושת שביעית?
Answer
There are certainly those (even who rely on the heter mechirah) who treat heter mechirah products as having kedushat shevi'it. (The custom of Benei Berak, those following the Chazon Ish, is to treat yivul nochri as having kedushat shevi'it as well).
Now, at the beginning of the shemitah year (with the exception of certain vegetables that have been planted during the shemitah year and now arriving on the market: basil, chard, chives, coriander, cucumbers, dill, lettuce, parsley, radish, strawberries, squash, and turnips and soon cucumbers), many vegetables were planted before the onset of shemitah and most probably began growing before the year began. As such, there is not a problem of sefichin, so even if you do not rely on the heter, eating this produce is not as problematic.
As you thought, you should treat the produce as having kedushat shevi'it if you do not rely on the heter mechirah. Just so you know, many vegetables, especially root vegetables, are still from the sixth year.
See our sefichin calendar here: https://en.toraland.org.il/beit-midrash/articles/shemitah/shemitah-calendars/shemitah-calendar-for-vegetables.
Follow-up question:
I have two further questions.
If I don't believe in heter mechirah, and the vegetables are from this year, isn't it going over איסור ספיחין? In which case, is it not somewhat bizarre to go over one איסור (ספיחין) while being careful to keep another (treating this produce with kedushah)?
Also, can you explain the halakhic basis for those people you mentioned who rely on heter mechirah and treat it with shvi'it? I should think it's an outright contradiction.
Answer
If you don't rely on heter mechirah, then indeed sefichin would be a problem (OU does not certify heter mechirah vegetables). If it is possible to avoid eating those specific vegetables that are planted during shemitah, that is halachically preferred.
If you don't have a choice, there are many rabbis (including Rav Ovadia Yossef) who did rely on heter mechirah and even some who ate heter mechirah vegetables (Rabbanit Margalit attested that her husband, Rav Ovadia did), so you do have halachic opinions to rely on.
Fruit is a separate issue, since sefichin is not a problem (there are the issues of שמור ונעבד, that is, they are guarded and forbidden activities are performed on their behalf). Many posekim are lenient for shamur/ne'evad for fruit. It is much easier to be lenient.
Kedushat shevi'it is a separate issue. The Mabit (and others) held that even produce owned by non-Jews has kedushat shevi'it. The Chazon Ish relies on this opinion. For a comprehensive discussion on the subject, see Shabbat Ha'aretz 4:29 §2.