For instance, right now the L3+ is sold out on Bitmain. While that would be ideal, its sold out.
For a 400-500 dollar premium, u could pick one up from a verified retailer like Eastshore.xyz
Whether that premium is worth it is up to each individual, but IMHO saying "only buy from manufacturer is a bit simplistic
Don't be stupid, there are only a handful of bitcoin mining hardware manufacturers. If you aren't purchasing directly from them you are making a big mistake.