Maasros Chapter 5, Mishnah 3. The next two mishnayos deal with the prohibition of "lifnei iver" - literally, placing a stumbling block before a blind person, and in practical terms: causing another person to sin in a situation where it is known, or at least suspected, that he is likely to transgress. Our case concerns the worry of selling produce to someone who does not tithe properly, thereby causing him to sin.
It has already been established that one may not sell produce without first tithing it, as discussed in Maseches Demai and elsewhere. But our Mishnah deals with a case where the plant is still attached to the ground: must we be concerned that the lemon tree will produce lemons that are obligated in maaser, and the buyer will not tithe them?
The text of the Mishnah:
"Lo yimkor adam es peirosav mishebau l'onas hamaasros l'mi she'eino ne'eman al hamaasros" - one may not sell fruit while it is still attached to the tree, once it has reached the stage of onas hamaasros, to someone who cannot be trusted to separate maaser properly, because of the prohibition of lifnei iver.
Onas hamaasros, as explained in the first chapter of the maseches, is the stage at which the fruit is considered edible to a minimal degree, and from that moment it can be tithed and the maaser takes effect. In other words, this is the point at which the fruit begins to be considered food according to halachic definitions, and this has ramifications in many areas.
After onas hamaasros: there are fruits on the tree that have developed to this stage, and so there is actual food present and not merely a theoretical possibility. One can point to specific lemons that the buyer will not tithe, and therefore it is forbidden to sell to someone who cannot be trusted regarding maasros.
Before onas hamaasros: there is not a single lemon on the tree that has crossed the threshold of obligation in maasros and that one could point to, and therefore it is permitted to sell him the tree, with no concern for the prohibition of lifnei iver.
And what is the difference between the two? After all, lemon trees will eventually produce lemons, and sooner or later the fruit will reach the buyer's hands. Indeed, the question stands. The answer is that Jewish society could not function in a reality where one may not sell fruit or trees to anyone, and such a situation would generate a great deal of ill will among Jews. For this reason a distinction was drawn between a case where there is actual fruit that one can point to, and a case where no such fruit exists yet.
This law applies in the shemittah year as well:
"V'lo bashevi'is l'mi shechashud al hashevi'is" - one may not sell a tree to someone who is suspected of violating the laws of shevi'is, and this applies when the produce has reached onas hamaasros; before that stage, the sale is permitted to anyone.
Shevi'is fruit has its own restrictions: one may not restrict access to it, there is a final date from which the obligation of biur takes effect, and it may not be harvested commercially. But eating tree fruit that grew in the shemittah year is not a problem at all, since the prohibition of sefichin does not apply to tree fruit, and even selling trees during the shemittah year is permitted. The prohibition relates to working the land and the like, as explained in Maseches Shevi'is.
The same logic applies here: one who suspects that a person will not properly keep the laws of shemittah should not sell him fruit whose access he will restrict and which he will not remove as required. But as long as there is no fruit on the tree, the prohibition of lifnei iver does not take effect. And all of this applies specifically to someone who cannot be trusted; one who sells to a God-fearing person may sell him the tree with its fruit on it without any concern at all, since there is no reason to suspect that he will transgress.
And if some of the fruit has ripened early:
What is the halachah when the trees as a whole have not yet reached the stage of liability for tithes, but a few of the lemons have already passed that threshold? Do these isolated fruits render the entire orchard forbidden to sell? The Mishnah answers that they do not.
"V'im bicher - notel es habikuros umocher es hash'ar" - "bicher" comes from the word bikurim, fruits that ripened early. The seller picks those fruits that ripened ahead of the others, and sells the remaining trees with their fruit still on them. Since he is not selling anything that has reached the stage of liability for tithes, the sale is permitted even to someone who is not trusted regarding tithes, and likewise in the Shemittah year to someone suspected regarding Sheviis produce.
In summary: This Mishnah establishes that the prohibition of lifnei iver in selling attached produce depends on whether the fruit has reached the stage of liability for tithes. Once it has, one may not sell to someone who is not trusted regarding tithes, nor to someone suspected regarding Sheviis; before that point, when there is no actual fruit that one can point to, the sale is permitted, for otherwise Jewish society could not function and hostility would abound. And if some of the fruit ripened early, he takes the early-ripening fruit for himself and sells the rest.
In the next Mishnah we will continue with the subject of lifnei iver in the sale of produce.