Very fair question. Much like finding a new apartment or even spouse online...pictures can lie. Expectations can be way off. Pictures, videos, descriptions and even reviews can be way off sometimes. It's common for people to hesitate when buying candles online because...well...you can't smell them! Here's a few tips to help you make an educated decision about purchasing a candle online.
Know your space. If you're unfamiliar with your home, how a candle and smell reacts in that space and how it will "fill" your room, read this blog post. (link to post)
Now that you determined what your expectations should be for your home size vs candle throw, choice of scent becomes the next question. Scents are a highly personal thing and it is impossible to know each person's preference. What is possible though, is being realistic about your preferences. If your candle collection naturally seems to consist of manly, musky smells...don't go out on a limb and try a flowery scent. Stick to your instincts. Sure, everybody's lavender candle is different and unique in its own way, but at the end of the day, it is still lavender.
Read the description. It might seem silly and sort of obvious, but read the description of the candle you are interested in. Candle descriptions are always vetted and written with intention. Their sole purpose is to help you understand the candles smell in as much verbal detail as possible. Never is a description written with the intent of "tricking" someone.
Know the season and mood you're in. Sure, maybe you have a wide variety of scents you enjoy. But be aware of the season and mood you are currently in. A pumpkin spice candle may smell lovely in the fall, but not quite as welcoming in the summer when your nose is expecting a refreshing lemon smell. Candles are a great way to transform your atmosphere season to season, so make sure you utilize that!
Don't be afraid to start small. Buy a small candle first and give it a try. Burn it in your office, bathroom or smaller space to see if you enjoy it. Then spend the money and invest in a larger size to fill your home.