Three bad things can happen:

1-) AGE - Some vendors take a branch or a root cutting from a "mother tree" that is, for the sake of argument, 10 years old.
After 2 years in a cheap mass produced pot, the cuttings are sold as "12 year old bonsai."
They are not.

2-) WRONG PICTURE - Some vendors put in small letters: "the bonsai you get is similar in size and age of the illustrative one in the picture". Which means you are NOT going to get the tree you are seeing on the vendor's website.

3-) 2D ILLUSION - The picture is from the real bonsai you are going to get, but...
A photograph is only a 2D rendition of the 3D tree, many times showing only front/back views.
Which means 2D good looking trees in bonsai pots are prepared to "look amazing in the picture", but either they don't have back branches (no depth), or they are a thick stump with a lot of foliage on top (to hide the ugly chop) and no ramification at all.

I think it's incredible anybody would spent sometimes thousands of moneys to buy something they didn't see in person.
I have just got an offer from a bonsai nursery, 450km away from my home, trying to sell me a BRT for 2.000 Euros. No pictures attached.
Amazing.