pregnancy tests

so out of curiosity, how accurate are dollar store tests? my plan was to stock up on those and if I got a positive on one I would buy a better test and check again. I have taken 2 of the dollar store ones and the first one said invalid and the second came out negative. I have been having some symptoms which could totally be in my head and me just really wanting that positive. but wasn't sure if if those were accurate or should I get a real test now even with no positive on those.