Jon and Kate Gosselin REUNITE at Yard Sale ... Gimme My Stuff!

Jon Gosselin showed up to his ex-wife Kate Gosselin's yard sale Saturday in Pennsylvania ... which can mean only one thing ... the guy wants to get some of his old junk back.

Kate was selling off old stuff like helmets and car seats with proceeds benefiting the Animal Rescue League ... and apparently J.G. was feeling a bit sentimental.

Here's the problem ... he doesn't have garage sale money anymore.