I've been buying selling Marlins over the last few weeks and I just paid top dollar for a 1952 336RC in .35, which would be a generally more desirable model than a 1970's T in .30-30 - in this case top dollar at a dealer was $500.
I think unless it's a rare model (.375 Winchester, for example), I rather doubt you will break $500.
Most guys will prefer a new gun, despite the reputations of the older Marlins. You can get new 336C's for under $500 on sale. $336W's are under $500 regular price at most stores.