Your Marlin 60 and AR-7 aren't going to bring much money in. Depending on what single-shots you're talking about, your may get decent money or nothing at all. I'd say keep 'em. Why sell them? It's not like your bank account will be overflowing with cash after the sales.
I'm activly shoping around for an 8mm Mauser. The ammo is relatively easy to find and cheap (as are the Mausers themselves - well, maybe not the pristine non-defaced German K98's).
http://www.sgammo.com/product/surplus/340rd-8mm-mauser-stripper-clips-made-romaniahttp://www.samcoglobal.com/rifles-m24.htmlhttp://www.aimsurplus.com/product.aspx?i...amp;groupid=209