I'm electronics, not Guest Services (maybe this doesn't count?), but I had a guy attempt to buy an Xbox One (or Xbone for those nerdy/gamer TMs out there) with super fake (they didn't even feel like real money) $100 bills. I ended up calling the LOD at the time (who is our STL and a pretty tall and tough looking guy), and the guy trying to scam the store ended up taking his "money" back and told me that he'll spend his "money" elsewhere. I was kinda shaken up by what happened for a bit since I didn't know if he would come back and try to rob me upfront or whatever (which thankfully didn't happen), but our main AP guy who I'm pretty good friends with was watching the scammer and his friends (one of which apparently used a fake $100 to buy something else at the front of the store) was super happy that the transaction didn't go through, which calmed be down a bit.
Aside from that, the only other real 'incident' with a scammer was a lady trying to buy a bunch of electric razors with a few fake $70 off coupons. I told her that I needed to call the LOD over to approve it since it was a big coupon, but she gave me a total b***hy attitude and left without paying. I gave a heads-up to the GSA (since we didn't have any AP/TPS that night) about her, and the GSA told me later that night that she ended up leaving her cart of razors/toothbrushes without paying. However, the only real reason I got suspicious was because the night before, some other lady did the same thing to me and I let the transaction go through. I didn't think anything of it until the LOD and AP (who is the same guy as my first story) told me that those were fraudulent coupons shortly after. I was kinda expecting to get fired that night, but they reassured me that I didn't do anything wrong nor was I in trouble and to keep a look out in the future (which I obviously did and will do for as long as I can).