It seems to me as another poster has stated, to consider renting chairs/benches as that would be a more economical way to go. With the price of a better grade of lumber, materials, tools, and your time, you may be spending more than you would get for selling those benches to a "rental company". Also, have you contacted the rental company that you (hope) will buy your benches? Unless the rental company gives you a written contract showing $ for each bench, you may be sitting on those benches for a while. Be safe.