Anything forbidden by the Torah and meant for food (דבר המיוחד למאכל), even though permitted for benefit (מותר בהנאה), may not be traded; the reason of the decree (שמא יבוא לאכול), lending against it, the חֵלֶב exception (יֵעָשֶׂה לכל מלאכה), the incidental case of the professional (נזדמנו · שלא יתכוין · מיד), כמציל מידם, and the merely rabbinic prohibition (Shulchan Aruch, Yoreh De'ah 117 — one se'if / 1 seif, ~7 rules)
כָּל דָּבָר שֶׁאָסוּר מִן הַתּוֹרָה, אַף עַל פִּי שֶׁמֻּתָּר בַּהֲנָאָה, אִם הוּא דָּבָר הַמְיֻחָד לְמַאֲכָל, אָסוּר לַעֲשׂוֹת בּוֹ סְחוֹרָה. וְכֵן אָסוּר לְהַלְווֹת עָלָיו. בַּמֶּה דְּבָרִים אֲמוּרִים? בְּדָבָר שֶׁאִסּוּרוֹ מִן הַתּוֹרָה; אֲבָל דָּבָר שֶׁאִסּוּרוֹ מִדִּבְרֵיהֶם, מֻתָּר לַעֲשׂוֹת בּוֹ סְחוֹרָה. וְהֶחָלָב, מֻתָּר, מִפְּנֵי שֶׁנֶּאֱמַר בּוֹ: יֵעָשֶׂה לְכָל מְלָאכָה. צַיָּד שֶׁנִּזְדַּמְּנוּ לוֹ חַיָּה וְעוֹף טְמֵאִים, אוֹ שֶׁנָּפְלָה לוֹ נְבֵלָה אוֹ טְרֵפָה, מֻתָּר לְמָכְרָם — וּבִלְבַד שֶׁלֹּא יִתְכַּוֵּן לְכָךְ.
Anything forbidden by the Torah, even though it is permitted for benefit (מותר בהנאה), if it is an object meant for food (דבר המיוחד למאכל), may not be traded; likewise one may not lend against it (take it as a pledge). — This applies only to something whose prohibition is from the Torah; but something forbidden only by the Sages (מדבריהם) may be traded. — And חֵלֶב is permitted, for it is said of it: יֵעָשֶׂה לְכָל מְלָאכָה (let it be used for any work). — A hunter / fisherman (צייד) who happens upon an impure animal or bird, or to whom a נבילה or טריפה falls, may sell them — provided he did not intend it (ובלבד שלא יתכוין לכך).
Shulchan Aruch, Yoreh De'ah 117:1LEVEL 01
Hebrew text of the single se'if (broken down into ~7 rules) with a fluent English translation. The principle of forbidden trade (סחורה), the extension to lending against it, the חֵלֶב exception, the incidental case of the professional (נזדמן) and the merely rabbinic prohibition, explained with modern practical cases (investing in a non-kosher business, pet-food).
📖 StudyLEVEL 02
In-depth pilpul: the inquiry into the basis of the prohibition — a rabbinic decree (שמא יבוא לאכול, Rashba / Beit Yossef) or a Torah-level law (פסחים כ״ג, "לכם — שלכם יהא", the Taz's objection); the rule of דבר המיוחד למאכל (Shach sk1, R"T, the חזיר); the "הוקש למים" exception of דם, and the boundary of נזדמן / שלא יתכוין.
📖 StudyLEVEL 03
Comparative tables (prohibition מדאורייתא / מדרבנן, trade vs. lending against a pledge, intentional / incidental), the exceptions (חֵלֶב, דם), golden rules, classic pitfalls (כמציל מידם, passing off as kosher) and memorization of the ~7 rules of the single se'if.
📖 StudyLEVEL 04
The practical halacha according to the Shach, Taz, Pri Chadash and Pitchei Teshuva, then the Sephardic poskim (Yabia Omer, Yalkut Yosef) and Ashkenazi poskim. Modern cases: investing in / owning a non-kosher business, pet-food containing נבילה, חמץ שעבר עליו הפסח. Note: the Shulchan Aruch HaRav does not deal with this siman — this is a level of psak, not "Daat HaRav".
📖 StudyThis is the heart of Siman 117: anything forbidden by Torah law that is an object meant for food (דבר המיוחד למאכל) may not be traded (אסור לעשות בו סחורה), even though it is permitted for benefit (אף על פי שמותר בהנאה). According to the Beit Yossef in the name of the Rashba, the reason is a decree: שמא יבוא לאכול מהם (lest one come to eat from it). Holding a stake in a business whose trade is in such a forbidden food falls under this prohibition; the exact boundary (purely passive investment, a minority share, the corporate structure) depends on the case. For your own situation, consult your Rav.
חֵלֶב is permitted for trade even though it is a Torah prohibition, because the Torah itself said of it יֵעָשֶׂה לְכָל מְלָאכָה (let it be used for any work): it explicitly permitted it for use. That is why one may trade in it (tallow, industrial uses). Likewise, according to the Pitchei Teshuva, blood (דם) was compared to water (הוקש למים) and trading in it is permitted. For your own situation, consult your Rav.
Yes. A hunter or fisherman (צייד) whose trade it is, who happens upon impure animals, birds or fish (or one to whom a נבילה / טריפה falls in), may sell them — בלבד שלא יתכוין לכך (provided he did not intend it). The Taz notes this is permitted only to the צייד whose profession it is, not to a private individual; and the Rama adds that he must sell it at once (מיד), without letting it fatten in his keeping. Note: something forbidden only rabbinically (מדרבנן) may be traded in any manner (Taz sk4). For your own situation, consult your Rav.