I have my FFL and buy and sell as a side business (day job is in manufacturing).
I buy almost exclusively from online auctions.
I bought over 100 firearms last year from probably 40 or so different auction houses.

Frankly, you’d be surprised how FEW problems I’ve had.  The vast majority are also FFLs and do things right.
Last year I returned only one item.  A couple 1903 stocks I bought that turned out having severe termite damage.  The pictures were awfully poorly lit in the bad areas.  The auctioneer was very accommodating.