I personally think you’d be coming out ahead to make the trade for the Savage. The value of SP101’s is recently and considerably inflated, so I think folks are pretty reticent to buy or trade a used SP at $500 value, whereas it’s much more simple in the nationwide online market to sell a good condition 99 for more than $500. If the guy takes the deal, you win big by my standards - but I wouldn’t take the trade in his shoes. Maybe the SP + $100 or more.
At my shop, operating for profit, I wouldn’t take an SP101 even trade for a Marlin 1894 or Win 94 unless I was simply in a market where leverguns didn’t sell well - which ain’t here! The SP would likely move well when listed for $550, cash price $500, over a $350-400 trade value. But I’m not sure I think many shops are getting Marlin 1894’s or Win 94’s for $350-400, and I don’t see any locally listing them used for $400 - most are $500+. The SP101 should move faster than the levergun, but you’re really trading nickels on their profit potential. Same deal here - I think you come out considerably ahead if a shop takes the SP for a Marlin or Win in good condition.