Before you go whacking burls off trees,
pick up a can of Anchorseal and a cheap brush or two, anything you expose sawing it off should be sealed to slow the drying process and prevent it cracking. You may want to peel the bark back and seal the outside as well. Some folks may, and probably will, suggest that latex paint works to seal them; in my experience it truly sucks! Anchorseal on the other hand, I have had zero problems with.
Oak burl can be iffy, according to those in the know; occasionally however, they are killer. Box Elder, Maple, and Ash are usually winners. There's no guarantee that any burl is going to be exceptional, UNLESS... It has lots of little limbs shooting out all over it. The little limbs everywhere are indicative of eyes in the burl. The more little limbs, the more eyes; the more eyes, the more character the burl has.
Burls are often sold by the pound, since most are round and lumpy, making it kinda difficult to calculate board feet. Some serious buyers will want to know weight and moisture content, so they can calculate losses on weight as it dries. You can pick up a cheap
moisture meter on Amazon, I saw one in Lowes awhile back too. Somewhere around $20 will get one that will give you a reasonable surface reading to offer to a buyer. It doesn't have to be real expensive.
Records help too... date cut, weight at time it was cut, moisture content. Check moisture and weigh it periodically, write your findings down. Most folks won't care; many buying to resaw and resell probably will.