Andrew Bickel, 39, was fixing a faulty light at the Catwel shop in Heath on Thursday as a favour for the cat re-housing charity.
He said he nipped outside to get something from his van and when he returned his tools had been sold.
"The lady in the shop was really upset," said Mr Bickel. "Fortunately I can laugh at it now".
Mr Bickel, from Heath, later posted on Facebook: "Doing a love job in a charity shop, put my tools down, went to the van, came back and the old lady on the till sold them for £1.
"Funny but fuming. Over £200 worth."
After his post was shared almost 90 times, an electrical company on Ocean Way, Cardiff got in touch and offered him a new set of tools free of charge.
"It was a really kind gesture," said Mr Bickel.
Mr Bickel said when he went into the charity shop on Thursday it was busy. He left his tools by a bookcase and went to get something from his van.
When he returned and asked where his tools were, he said the lady in the shop "put her hand over her mouth" and said they had just been sold.
She told him a man came to the till saying there was no price tag on them, but he had found them in a bargain bin.
"He must have known what he was doing," said Mr Bickel.
The £200 tools included a wrap of professional screwdrivers, an expensive pair of pliers and wire cutters and an electricity tester.
"I was laughing yesterday and am still laughing today," he said.
Mr Bickel even admitted that he is allergic to cats and was "more of a dog person".
A Cardiff charity shop mistakenly sold an electrician's £200 tools for just £1 after he left them unattended.