Affiliate Marketing Secrets

I’ve got a ton of low/niche demand items like old books,cheapo gadgets/electro, clothing, that I get for almost nothing and mostly leave to auction with shipping padded to cover minimum acceptance rather than pay jacked up inserts and reserves (buggers get enough $ as is)
Anyways if I ever wanted to have a sale like buy one choose any other auction free for me to take down and ship with your purchase (for old stuff that isn’t moving) how could I go about putting my paragraph at the top of my description for all of my listings?
Manual simply is not an option for this. Free programs/sites like auctiva used to be preferred but would pay if I had to for the ability to on/off en masse. Over 2200 listings any given time that are real profitable -but move slow as crap.
And please don’t say buy it now or anything stupid like that. Inserts x thousands add up. Not to mention on old books and crap auction is vital for not knowing its worth. Some of the crap in the dime a pound pickings you get randomly carries 40 or 50 dollar demands…weird and impossible to expect in larger volumes.