I know this sounds fundamental but a lot of people have lost a lot of their own fish through not quarantining their fish correctly or adding to their ponds too soon.
I have taken a lot of advice on the subject and I now know that you should always quarantine your new fish for a minimum of 9 weeks and the temperature should be increased gradually until it reaches the optimum level for the parasites and infections therefore any diseases will show before you introduce the fish to your existing stock. If you need further help and advice try your local koi dealer or search the net for the koi health sites some even have videos to help identify infections and parasites.
It can be a very costly exercise if you only quarantine your fish for the 4 weeks suggested by some ebay sellers, as I've learnt to my own expense.
Please don't let this put you off bidding on Koi, Sturgeon, Shubunkins and Goldfish as there are some good reputable sellers out there with a wealth of advice, but please read feedback before buying.
If you cannot view feedback as it is private!
Just ask yourself why?
What would a seller have to hide?