Buying a used bike is a great idea if the price is right. If it was a good hardtail then $140 is a pretty good price. But, I wouldn't put another $350 into a used bike. There was no mistake in buying the bike, but putting more than the purchase price into it was the mistake. I've flipped a fare number of bikes before I took my most recent job. Any time I had to put more than $50 into a bike I would question why I was buying it. If I bought it and it needed more than $50 in repairs, I would question if I should cut my losses and just junk it.