People use apps most of the time for convenience but don't worry because even though localbitcoins don't have their official app yet. The site's mobile version is very responsive and has a nice look and feel. Have a look at this, I tried it on different mobile browsers but its looks the same. I use chrome in that
Just don't forget to always check if you're in the right site because there are many phishing sites of localbitcoins. Its usually found on the promoted sites on google if you search "localbitcoins".
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I know some legit apps on play store [well they also have their web version] but they're a local exchange so it depends which country you reside in.
Can you tell us which one?