As someone who actually used to work for a bike shop, I actually think that's a good idea. Most employees would appreciate the honesty and would let the "leech" throw his legs over a few bikes as long as he doesn't damage them. A smart shop manager would make sure to offer a discount on the used bike's first tune up in order to ensure a repeat customer. College students stand to be your customer and/or advocate for many years to come. Plus, they are likely to make enough money to buy your top rack bike a few years down the line.