so its 100$ buy in and I have around 300$.

everyone folds except the button who makes it 15$ with QJoff and I re-raise to 40$ he calls pretty quickly and flops Qs-7c-5c. I'm first to act and bet 50 and he calls fairly quickly. turn is an 8d and I bet another 75. He takes about 10 seconds and calls. River is another 8s and I jam all in with about 150$ left. He takes about 15 seconds and calls. Could I have made this bluff better? He said he put me on AK cause I three bet to 40$...

You have to remember that low limit players don't like to fold often. They would rather say they think someone has AK rather than a big pair cause that's their justification for calling. Just learn to only bet for value on these lower limit games. Try not to bluff.

You could have easily had AA KK QQ or AQ that went for 3 barrels and he would have just folded and not said a word then fold. These low limit players justify bad plays however they can and when that lose it was either "I had to call" or "it was just a cold deck" or " he is a lucky bastard" perhaps a flop bet. Then turn and river check. If he is calling flop light cause he thinks your light with AK then you have the nut no pair. Which people still don't get.

Calling your preflop 3 bet and flop bet. He almost always has a queen or some type of draw with pair or combo draw. Which he isn't likely folding on turn or river. So best to shut down and check turn.

But remember since your going to take this line with AK. You need to sometimes take this line with AA or KK. Meaning cbet. Then turn check raise. If he checks back. Usually bombing river to look like steal that you haven't given up. Always have to merge your ranges