Learn Turkish: Antika eşyalar diğer pazarda satılıyor. - Antiques are sold at the other market.

To say "Antiques are sold at the other market, aren't they?", you simply add değil mi? to the very end: Antika eşyalar diğer pazarda satılıyor, değil mi? Even though the subject is plural (eşyalar) and the verb is positive, this tag is invariable and never changes to match them.

How is the passive voice formed in the word satılıyor?

The root verb is satmak (to sell). To make it passive ("to be sold"), we add the passive suffix -ıl, making it satılmak. Then we drop the -mak and add the present continuous suffix -ıyor to get satılıyor (is being sold / are sold).

Why isn't there a plural suffix on the verb satılıyor since the subject is plural (eşyalar)?

In Turkish, when a plural subject is non-human (like eşyalar - items), the verb is almost always conjugated in the singular third-person form. So we use satılıyor instead of satılıyorlar.

Could I just say antikalar instead of antika eşyalar?

Yes, you can. Antikalar means "antiques" and is perfectly understood. However, antika eşyalar (literally "antique items" or "antique goods") is a very common, natural way to refer to vintage furniture and objects in Turkish.