If the salesman sold you a bike where you can not firmly plant your feet on the ground while seated on the saddle he did not set up the bike correctly. If it is still the case after adjusting the saddle height correctly for your leg length (knee just slightly bent at the maximum extension) then he sold you a bike with too large a frame. Appears to be someone more interested in making a sale than meeting the customer's needs correctly. I was going to suggest you look at used bike which is heresy for someone who long ago worked in a bike shop. There are many quality used bikes for sale. Often they have may have had very little use by the first owner. It helps if you can recognize a quality bike from a junk bike if you chose that route. Now days you can go online and get specifications, owner's reviews, and original prices for most of the common used bikes.