I assume you're asking about Gateway Classic Cars.
I've heard stories about people finding out after buying from them that the car was not what it was represented to be. Their standard response is "We sell on consignment and the description is the owner's."

Definitely a case of buyer beware. I've heard numerous stories of people buying consignment cars and being very disappointed. Not specifically with Gateway, but consignment in general. I'd read their contract really close and see what if any recourse the buyer has on you if they are unsatisfied or have issues.

People buy classic cars all the time then find out the car wasn't what they thought.  Whose fault is that though?  You're dealing with a 50+ year old vehicle.  Its entirely possible that the seller doesn't even really know the condition of the car.  If you're not going to go over the car thoroughly, or if you don't have the skill to do that, take it to a trusted place to do it for you; then you get what you get and honestly shouldn't grouse about it. 

As far as commissioned sales being worth it, it depends.  A 12.5% commission seems high, but let's say that you sold the car for $30k instead of $20k, even after commission you are still $6,250 ahead.  Also, how quick can they move it?

There's a lot that goes into that decision.  What's your time worth is a major one.  You can definitely list the car on eBay, or one of the dozen or so other sites that lists cars and get national or international exposure.  Even sites like Craigslist can be effective.  The big question is, how many tire kickers are you going to have to entertain?  How many schmucks are going to roll up and try to offer you half your ask because they have cash (yeah, like I was going to take a check from you...). 

Classic cars to most of these places are commodities, sold with the same emotional attachment as oil contracts on the commodities exchange.  They are a means of making money.  99% of those places are going to be at least somewhat shady.  I mean, they're selling used cars after all.  Albeit classic used cars. 

Id try Craigslist, Ebay and Facebook Marketplace first!