Breakdown of furimaapuri o tsukaimasu.

Questions & Answers about furimaapuri o tsukaimasu.
フリマアプリ is a compound noun:
- フリマ = short for フリーマーケット (flea market)
- アプリ = app
So フリマアプリ means a flea market app, usually an app where people buy and sell secondhand items.
を marks the direct object of the verb.
In this sentence:
- フリマアプリ = the thing being used
- 使います = use
So を shows that the flea market app is what the speaker uses.