Are fish bought and sold then Shipped to you safe or are they stressed or even dead on arrival?
All of the above.
They can be good and they can be bad. Some suppliers are better than others but in general, most will try to send you healthy fish.
Ask for reviews from people who have used specific sights. If you get 10 people saying a sight gave them great fish, try it and see.
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Avoid buying fish, plants, shrimp or snails when the weather is very hot or very cold. Cold, icey or snowy weather can cause delays in the shipment being delivered to you.
Don't buy fish at the end of the week because some places send out orders as soon as they get them and fish can end up sitting in a depot over the weekend. Don't buy around the public holidays for the same reason. If in doubt, contact the company and ask them to send the fish on a Monday.
Make sure the fish are sent express 24 hour delivery. The less time the fish are in transit, the more chance of them arriving alive and well.
Make sure someone is home to receive the fish so they aren't sitting outside on a freezing porch or cooking in the midday sun.
Take pictures of the box and fish when you first get them. Clear pictures with a time and date stamp, that show a dead fish in a bag, can be used as proof a fish died during transit. Likewise, clear pictures with time & date stamp can show damage to the box, if it was damaged during transit.
If you have to make a complaint to the store, do it in an email so there is a written copy. Be polite and inform them of your details, account number, fish, etc. Then tell them x fish was dead in the bag when you opened the box, and you have attached a couple of pictures to confirm the dead fish in the sealed up bag.