Of course from the buyer's perspective there's a potential issue of relative levels of trust. If you're offering your bike for sale for $500 (to pull a figure out of the air) and the buyer leaves you $500 in cash as collateral to take it for a test ride then you've got about the same level of trust in each other. Presumably by then the buyer has examined the bike and is happy that, subject to a test ride, it's worth the $500 you're asking.

But just as you wouldn't let the buyer take your $500 bike for a ride with only $50 in collateral so the buyer may not be happy to leave you the keys to his $3000 car as collateral for the $500 bike. I know it's not quite as simple as this but just as the buyer wouldn't leave you $3000 in cash to take your $500 bike for a spin so he might not be happy to leave his car keys as collateral. As the seller you want to be sure the buyer will be back, and the buyer wants to be sure he's not going to come back on the $500 bike to find you've cleared off and taken his $3000 and left him with the bike.