Bonsai yellow leaves question

[Rajned]

Hello all I’m new and have a question regarding the tree I bought from ( don’t know if I can say the hardware stores name ) it was 128 dollars marked down to 20 doesn’t say what species but I’m guessing a buxus.

The tree is still alive and a healthy green when doing scratch test on branches. I checked if it was root bound and it’s not, soil drains very well and I see no signs of any bugs.

Worker there told me however that the tree was out in the full sun next to corrugated iron and that it was not watered then overwatered for a long time. I’ve looked a lot up but I’m just wondering if anyone here could perhaps give me any ideas to hopefully get it to recover if it’s even possible.

Thank you in advance for taking the time to read and any help given.

[Rajned]

Sorry the trunk picture wasn’t clear so took another

[shibui]

The bits that are alive look healthy and strong so I'd say it has a good chance to grow on. Hopefully most of the trunk and roots are OK and you'll have a nice start to a bonsai.
I'd say lack of water caused the problem. General nursery staff don't seem to appreciate how often plants in small pots really need to be watered. Over watering is far less likely to have caused problems.
Hope you can do a better job and keep it alive for the rest of summer. Buxus are not usually fast growers but a larger pot for a couple of years would allow room for roots would allow it to recover and grow a little.

[Rajned]

Thank you very kindly for the help, hopefully it pulls through.