I don't sell often so I don't have much to say on that part, but I do buy a lot. Like a lot. If I don't buy at least one wolf a day something is wrong.
My limit for wolves solely depends on what I'm buying them for.
Pups that I intend to level passively through companions for future release? 1000p limit, I often find them sold for less. No point in spending more, if I check back in a few hours odds are there will be more for 1k or less.
Adults that I intend to release? I try to cap it out at 5kp, I like to stay under 2kp. I don't want to buy wolves to release that someone intentionally priced higher to avoid that. But I am selectively choosing which wolves to buy and release and sometimes I spend more than 5k. My absolute max for buying wolves to release is 10k pebbles, because at that point I'm only profiting the knucklebone
Wolves I intend to keep? Well. I'll shell out a lot of moonstones for a good wolf. One with a special ID, or a low digit, or is a bracc. I'm embarrassed to say how much I've paid for some wolves. For some reason I'm significantly less willing to pay a lot of pebbles for a wolf I like, though. Once I see a wolf go above 50k pebbles I'd better really want it. Anything over that and I can either wait for someone to undercut, or get a similar wolf for 1 or 2 moonstones.