You can mark them up as much as you like (and can get away with!!) but the typical practice for most pros is to purchase wholesale, then mark up to retail. If you haven't established any business or wholesale accounts with vendors, you should do so - most only require proof of a related business (licenses, etc.) to set up. It is a very common practice and an economical bonus for any landscape professional. The cushion between wholesale and retail pricing is part of your income stream.

And they are accounted for as an expense of doing business - "supplies", if not a more detailed category.