I can't emphasize this enough, be specific about what you are looking for. Try to detail out the strengths you are looking for. You can mention what your own rabbits have for strengths and weaknesses.
It's helpful to say you are looking for a great head (aren't we all?) but explain what it is you want. Is it more fullness in the cheeks, a more pronounced brow or more side profile curvature? Without the details, a seller may have in mind a big head but with less definition and sharper angles than you want.
When requesting a good crown are you wanting more side to side width, more depth of crown or more definition?
Don't be shy about money, state your bunny budget. Don't worry that a seller will UP the price on a rabbit simply because you gave them a dollar amount higher than what they were going to charge. Because ultimately YOU will be looking at that rabbit and deciding if it is worth what they are asking for it. They aren't "holding the cards" because they know what you will spend, you are "holding the cards" on deciding whether to spend it. If you want to spend no more than X number of dollars for a buck or a doe, say so. We all have our spending limits that we can justify in our heads....or our spouse's. LOL
List particulars such as age, color, size or bloodlines. And don't get hung up on the latter. Many times the bloodline is a piece of paper, not a piece of rabbit. I can't say it enough, decide on the rabbit sitting in front of you, not that it is a son or daughter of the famous "so and so" rabbit. Some apple heads fall farrrrrrr from the tree!
Always ask the seller what weaknesses they think the animal has. An honest seller will be upfront and really upfront sellers will give you their opinion of strengths and weaknesses without your having to ask. There is always a reason a rabbit is for sale. If there wasn't, it wouldn't be for sale, right? No one is so overrun with perfect herd bucks that they have a lot of other perfect herd bucks to spare/sell.
Remember look at the animal in front of you because ultimately that is who you are buying. You aren't buying a photo sent pre-convention. With that said, if you make a deal before convention based on a photo make sure it is "pending upon approval". This means that you will buy the animal if it lives up to the seller's description and to its' photo after a hands-on examination by you. On that same note, connect with the seller as soon as possible but not upon arrival while they are trying to cooop their rabbits. Go over the rabbit with them and try to make your decision right then and there. If you need time to think about it, tell them you will let them know within a couple of hours. DO NOT leave the seller or potential other buyers hanging. Animals sell quickly the first day or two. Don't be too embarrassed to tell a seller the animal isn't what you were looking for, the sooner they know the better. Just as there are "bad sellers" there are "bad buyers" and word gets around in the HL world.
Ask to see related animals entered at Convention. This may give you clues to breeding consistency potential.
Pay promptly and get the pedigree right then and there. Don't hand over the money and leave convention without a pedigree. Also, get a sales slip immediately because you won't be able to take that animal out of the building on check-out day without a sales slip. Put it in your wallet because it is as good as money! Write SOLD and your initials on the coop tag. Ask the seller for some grain if different from what you feed. Trying to track down folks at convention later isn't easy so try to get these three things as close to the sale as you can. Remember... pedigree, sales slip and food if necessary.
Best of luck in your search. You surely will find that special animal that will help improve your herd and keep you inspired to stay in Hollands for many years to come!