My experience was the flip-side as I was trying to sell but i'll add it in here anyway:
I started the process of selling my old Chevy Spark to Carvana plus a couple other online places. In the end, I'll echo Clay's comments of, unless you are VERY sure don't bother. They try to get people to move fast and it's a business model that preys on this sense of urgency but after you sign off they take their sweet ass time. After it was said and done, I chose not to go through with the transaction with those online places primarily because when I kept pressing for a timeline, they said it may take "maybe a month or more" for me to get paid for the car. Personally, I think some of these online places offer an inflated price to people so they feel pressured into a higher sale price but they drag their feet paying you until they find a buyer that will cover that inflated price.
I took a thousand less at the dealership because it was was a much more confident and solid transaction.