I received an associate offer with no buy-in option. Why would anyone agree to this? What should I ask during negotiation?
Sounds like the owner has no intentions of retiring anytime soon, and wants to keep the bulk of the profits to him/herself and is looking for an associate to take some of the volume off of them.
I would personally be very wary of an agreement like that, as chances are you'll be seeing only certain types of patients that the owner may not really want to see, and not have much autonomy in the practice.
If you do choose to work in that practice, I would make sure that you get as many things written into your contract as possible, from pay, potential bonus clauses, benefits such as CE, who's paying lab fees, and even a right of first refusal should the owner decide to see in the future with some specific wording about your effects on the practice production and how that figures into any potential selling price. Heck, I would even try and get something on paper that should the owner sell to someone else, that if you want, and are still employed by the practice, that you have the option to continue on with that practice and any potential future owner, should you want to.
If the owner balks at things like that, then IMHO you're looking at a situation where the likelihood is you're probably not going to be satisfied with the work arrangements you'll be walking into