Bulk account sellers offer deals for Wintertodt HCIM these days, plus people wanting that type of account would prefer to just get it serviced by an account made by themselves, so the price fluctuates pretty frequently from $50+
Assuming you've gotten 99 after getting the full Pyro set, it wouldn't necessarily increase the price as the skill's already done at that point - all it does is offer 2.5% boosted XP...
Acquiring the Phoenix pet usually increases the overall value, but again it's dependent on the buyer
The broken record saying is the more reputable and trusted you are, the more likely you are able to profit in the overall price for an account