käuflich
available for purchase
Such that one can buy it; offered for sale.
Diese Software ist nicht frei verfügbar, sondern nur käuflich zu erwerben.
This software is not freely available, but only available for purchase.

available for purchase / corruptible
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available for purchase
Such that one can buy it; offered for sale.
Diese Software ist nicht frei verfügbar, sondern nur käuflich zu erwerben.
This software is not freely available, but only available for purchase.

The adjective 'käuflich' (available for purchase/corruptible) is derived from the verb 'kaufen' (to buy) and the ending '-lich'. It literally means 'buy-able' or 'something that can be bought'. This basic idea applies both to products and to the (negative) description of people.
Ist dieses Kunstwerk käuflich oder nur eine Leihgabe?
Is this work of art available for purchase or only on loan?
The meaning of 'käuflich' (available for purchase/corruptible) depends on whether it refers to a thing or a person. If it refers to a product, the meaning is neutral. If it refers to a person, it is strongly negative and means 'bestechlich' (corruptible/open to bribery).
Die Eintrittskarten sind ab Montag käuflich.
The tickets are available for purchase starting Monday.
A very common and somewhat formal combination is the phrase 'käuflich erwerben' (to purchase/acquire for money). It is often used in business language or official information.
Das Buch können Sie in jeder Buchhandlung käuflich erwerben.
You can purchase the book in any bookstore.
trade
'der Handel' and 'käuflich' are connected when it comes to something being available to buy. 'im Handel' means that a product is available in stores or generally on the market. 'käuflich' is an adjective and means that something is for sale. Learners may confuse the words because both have to do with buying. Also important: when used about people, 'käuflich' can have a negative meaning, suggesting that someone acts dishonestly for money or is open to bribery. 'Handel' does not have this meaning.

Das alte Haus ist käuflich, aber der Preis ist sehr hoch.
The old house is for sale, but the price is very high.
Diese Sorte Tee ist nur im Handel in größeren Städten zu finden.
This type of tea can only be found in stores in larger cities.