You could say the same about getting fuel for your car at the pump?
How do I know I'm actually getting a litre or ... ?
Like all walks of life, we are always open to being scammed.
We can only do so much to protect ourselves. Going with a reputable company is the best that can be done in this situation.
Me .... I go with a reputable company, nut I'm always there to accept delivery (after work or on a Saturday) and I can see what the clock says before and after.
Other than that I can do no more.