Question about practice contract

[Sam Garner]

Hey guys, thanks for your time. I'm doing some practice scenarios before I take my Real Estate license test, and there's one thing I still am having a slight issue wrapping my head around, which is representation. Here's the scenario. (This is Texas, btw.)

Tim lists property for a couple, then agrees to act as the selling agent for them. 

Sam, a buyers agent, brings a couple he's been working with to the table and they put down an offer on the property. 

On the form, I'm a little confused because Tim listed property, so is he not the listing agent? But he's also acting as selling agent, so isn't he the selling agent? But the material I've read says that if you fill out the listing and selling, that's all you should fill out, but what about the buyers agent? I'm obviously completely missing something, and idk if there's more I need to add to this to make it understandable for one of you, but I'm quite confused on this one thing. 

Thanks guys