Fill in the blank with the most appropriate word.
Second-hand furniture ______ here at reasonable prices.

There are some uncountable nouns (followed by a singular verb) that don't take the articles 'a/an', many, few, and plural form:​
Furniture, scenery, luggage, temper, hair, knowledge, equipment, pottery, music, jewelry, etc.​
Example:
​The sceneries at my friend's house enchanted me. (Incorrect)
The scenery at my friend's house enchanted me. (Correct)
Since the noun 'furniture' precedes the blank part of the sentence, 'is sold' will be used as per the rule given above.
Correct sentence: Second-hand furniture is sold here at reasonable prices.