Sounds like a bacterial or viral  infection to me caused by columnaris.

Symptoms are the following:
white fluff blowing in and out of mouth, dry skin, skin color darkens, white  fungusy or fuzzy looking sores, red anal vent, chronic bubble eating
check for mouth and anal vent for sores; slime coat; mushy belly; gills light to dark brown patches, white sores

You can treat with medicated food ( antibiotics ), potassium permanganate in the water, injection of antibiotic if possible. Also topical treatment of wounds.

Improve water conditions, quarantine new fish.  It almost always indicates that a parasitic infection is going on and an accumulation of organic debris, including fecal matter. So please do waterchanges every day about 10% and continue treatment with Melafix AND Pimafix.

Sometimes those poor fish at markets and shows, carnivals are not in best nick, unfortunately. Sometimes the owners just want to get rid of them because they do not want to care too much, it became inconvenient or so.

I've been on a market the other day, they had a few tanks with goldies, one fellow was a pretty big boy, but it seemed to me that he had fin rot... poor thing

What kind of tank do you have?
Did you test for all water parameters like Ammonia, Nitrite, Nitrate, pH?
Do you have good filtration and aeration?

Do you use a water conditioner when changing the water? It's like a chemical that neutralizes toxic substances in raw tap water. Like chlorine and chloramine.
Bad water is usually the cause of most fish diseases since a fish is usually battling several bacteria and viruses at a time, but their immune system can usually handle these until some environmental problems raise the level of stress on the fish, weakening it and making it more susceptible to infection.

Get yourself a nice 20 gallon tank for the fella, if he's a common goldfish, he needs all the swimming space he can get (: And it's generally not good to keep a fish in such a small space, especially in case he might be in a bad condition from before. But just keep an eye on this dude!

Just in case, it could be good for you to buy yourself a few more text kits, for ammonia, nitrite and nitrate so that you'll be prepared for the bigger tank and the cycle! (: