Key is knowing what you are buying. So I ONLY buy from "this is the tree you get" places. Mostly from private sales, rarely from stores (But then again.. Kaizen bonsai has good stuff and I order from there too).
Make sure you get clear pictures
If you buy from private, make sure they are RECENT pictures
If you buy from stores: Make sure they have a a good reputation
Ask how they will package the tree. Just tossing it in a cardboard box only works for the smallest of trees.
Accept that banchlets may break and the pot may not survive.
The majority of my collection came from online. As mentioned above. I totally stress buying a tree you see pictured. Not a "comparable " one of the species. To not know what is being sent...is like brown bag prize bags I would buy as a child at the flea market. With our hobby having focus from roots up. Its important to me...for the most part to know what I am putting my money to.
I also like images of a dormant tree or defoliated tree. To see structure so nothing is hidden. Not all can supply those images. If they can't...then something about the character bones of the tree best inpress me. Or I pass.
There are several respectable sellers...not knowing your location. Has me pause sharing links. That takes time...and if you aren't in the states. Its a waist of time.
I've bought about half of my collection online but its the more significant half...
In Minnesota we don't have bonsai nurseries, I have taken to trying to encourage friends and other club members to open one and assure them that I will be a customer.
my tip would be to search the suggestions of bnuts and then look at those businesses, some like Andrea at Wigerts are happy to answer the phone. Generate a good selection group of potential growers and dig in.
My fav online seller I mentioned in a previous thread of mine, bought several trees from him over ebay, now we text and I get a discount, after reaching out he was more than happy to be more personal