How to get money back from Colleague?

Two weeks ago a colleague from other department (Our company has 400 employees and I don't need to work with him) posted an ad on our company's internal website looking for a used iPhone. I happened to have one for sale and I emailed him. He came to my desk and agreed on the price $150. He also asked if he could try it for a day to see if it's suitable for him. And I agreed as I thought that's reasonable if he had a few options.

The other day I asked him if the phone is OK. He said everything is fine and he is going to pay me the same day. That was a Friday. He didn't show up that day. Then after the weekend, on Monday I sent him email asking him again. He said he was busy but he is going to pay in the afternoon. Again he didn't come. On Wednesday I asked him why he didn't show up. He said he was sick and he is going to pay on that day. It have been a few times he promised to pay but never came. It has been two weeks. I didn't want to go to his office to ask for money as I don't want to interfere the people in his office with their works.

So how can I get my money back? Will HR normally take care of personal issues(it happened inside the company though) like this? Or should I talk with his manager?

Added (1). So if it's still my property, and he is keeping my property with the intention of depriving my owner permanently of its possession. Can I say this is theft and talk with HR?

I don't think his manager or HR will help you unfortunately.
I'm not sure but I really don't think so.

I mean it's uncomfortable for you that he did not pay, so you could go up to his desk and explain the situation to him in front of coworkers. Tell him you've been waiting for it so long and either way, he gives you the money now or just take back the phone. It's still your property!

HR & management aren't going to help you because this is a personal mattr not a company one.

you need to tell him you want the phone back if he's not going to give you the money & if you don't have the phone back or the money for the phone by x day & time, you'll report it stolen. (you'll need to do this with both the carrier & the police. The carrier will shut the phone off so it's useless to him & he can deal with the police)

in the future, I don't recommend you allow trials on merchandise you're trying to sell. Just tell them it's as is & all sales are final.