Could be worth up to $500 for an immaculate, all matching rifle. It replace the Mauser Model 1871, used a 7.65x53 round nosed cartridge, variants of the rifle were also adopted by Turkey and throughout most of South America.
50-200 USD or so
$15 to $40 depending on condition.
Depends on condition and originality. Anywhere from $100 to $275
5-50 usd
50-500 USD
Find a gunsmith and ask for help
$275-$450, depending on EXACT model and EXACT condition.
Gun shows, e-gunparts.com, want ads.
Leave it as is.
It would depend on the type. There was a rifle, carbine, and engineer's model. If it was in good shape, I would say that the rifles would go for $150 to $200, the carbines for $160 to $210, and the engineers (which are fairly rare) for $250 to $300. Many of these rifles were sporterized and that would lower their value somewhat.
It may not even fit an 1891 Mauser. Bolts are not a "drop in" part- they may require fitting (headspacing) to that individual rifle. That is where a gunsmith earns his pay.
7.65x53, also known as the 7.65 Argentine or Belgian.