This prevents sellers from over-bumping their ads to the top and supplanting newer ads. 72hrs seems ok as a limit but let me know if you think this should be increased.
For clarity, if you wish to add information to a listing, respond to a post in the listing, or amend the listing in some way, such as a change in price, the "bump" limit is not intended to restrict that activity.
No restriction bumping, one week bumping or 72hrs meet you half way bumping? Three days or 72hrs is very fair but I would eliminate the respond part cause it will be abused.
For clarity, if you wish to add information to a listing, respond to a post in the listing, or amend the listing in some way, such as a change in price, the "bump" limit is not intended to restrict that activity.
The other change (just IMHO) is to require that posters combine for sale ads. Other forums do this so you can create a dozen ads for a dozen separate items.
I've monitord the ads for a while now and it seems that bumping within a 7 day period is quite rare anyway. So @Sirius Glass it seems your proposal solves a non-existent problem.