I would opt to install myself but “dangerous” is a relative term. Its definitely more dangerous than installing yourself since you can’t verify their install source or process. You can factory reset when you get it but again can’t validate the Rom used as the reset image.

If risk is a concern of yours, I’d buy used and install yourself. Not only will you save money, you’ll know you installed from a legitimate source.

The install is so easy for graphene, Theres so little risk of messing up. It takes less than 10 minutes.