Learn Farsi: او همیشه وسایل دست‌دوم می‌خرد.

The yek ... -i structure is for a single, specific but unidentified thing (like 'a certain item'). In this sentence, vasāyel (items) is plural, and he buys them as a general habit. For general plural objects, Farsi just uses the bare noun without yek or -i.

Is وسایل a plural word? It doesn't have a standard plural ending like -hâ.

Yes, it is a 'broken plural' borrowed from Arabic. The singular form is vasileh (item or device). Instead of adding a standard Farsi suffix, the word's internal letters and vowels change. As a B1 learner, you will start seeing these Arabic-style plurals more often.

How does the phrase for 'second-hand' translate literally?

The word dast-dovvom literally translates to 'hand-second' (dast means 'hand' and dovvom means 'second'). Just like in English, it refers to something previously owned. Notice that it connects to the noun using an ezafe: vasāyel-e dast-dovvom.

Why doesn't the direct object take the -râ suffix here?

The suffix -râ is only used for definite direct objects (like 'the specific items' or 'those items'). Because he is buying second-hand items in general, the object is indefinite and generic, so it does not take -râ.