Yes lots of times. Best so far paid £1 for an Isle of Wight perfume bottle sold it on Ebay for £125. Bought a Gucci watch as part of a mixed box at auction for £20, paid £45 to have it repaired and sold it for £280 on Ebay. I find there are few if any bargains anymore at Charity shops as the bigger ones now employ people to cherry pick the good stuff and the charity then sells it on Ebay. Also I would find it uncomfortable to make a huge profit out of a charity item.
I bought a vase 30 years ago for 50p. Turns out to be a
Charlotte Rheade. Not worth mega bucks but when I took it to the Antiques Roadshow in Oldham, Eric Knowles said it was worth £80. I've still got it but my guess is that it's worth less now, having looked them up on tinternet.
I wouldn't put any store in that ebay price, Retro. There is a 50p coin for sale at £2,500. Next to it, the same coin design is on offer for £1.99. I suppose an item is worth what someone is prepared to pay.