It's a mostly young audience reading 280-character chunks of info making their user attention span very low. Whatever you choose, it's your style of interaction in the account that matters the most. Ask questions, make polls, make clever jokes in your ads, tease your followers with exciting tweets and language, and truly engage with them. To find the best selling products at the very moment though, you need a good tool that gathers all the latest ads in the most comprehensive, up-to-date way (twitter trends change really quickly) and shows them to you through different features like engagement and placement. so yeah, an ad spy tool should help you figure out exactly what niches are doing good.