What does "legit" mean to you? Some companies will list properties that they don't actually represent and try to extract an extra commission by getting in the middle of the deal. I'd consider those illegitimated. Some mainly deal in other timeshares or different types of property, but get a
DVC listing on occasion. They might know how to process the deal, or they might stumble through it and take much longer than a more experienced company. They may have a legitimate listing, but the experience may not be as smooth. Some charge ludicrous closing costs. Again, not illegitimate, but not exactly transparent on the pricing.
Ask them how many DVC deeds they've closed in the past month, and ask for a full accounting of costs with fees.
I tend to own specialty vehicles; cars with low production volume. When I need to take it to a shop, I ask if they've worked on this particular model before. If they can articulate some of the nuances of working on the vehicle, I trust them. If they start listing off all the "fancy" vehicles they work on and avoid answering my specific question, I know they're not the shop for me. Ask questions that prove they know what they're doing. If they say they close "hundreds of deals" but can't list their DVC experience, you'll save some headache and heartache by walking away.