R51d Foreign Exchange Advice

I am considering placing this stamp for sale on E-bay. It appears to be an un-used stamp that was re-gummed. Some of the gumming material has made it on to the face in lower right quadrant. I have a couple of questions regarding it.
1. Should I soak the stamp and see if the gum stuck to face improves the overall appearance of the stamp, or leave it for the potential buyer to decide?
2. Being a fairly scarce stamp, what would a fair starting price be? Just a ball park figure, not asking for a set value.

I have a second one in my collection that is much nicer with an 1871 cancel. I am wondering if it is even worth selling at this time. Seems that the market has been a bit lull for revenues lately. Either way, here are scans of front and back. The single piece of silk is visible front the front as well. Thanks in advance for any opinions. I would love to keep it, but my car is needing some repair.

I doubt if it was regummed, it was probably put on a document and then popped off later. That is not all that uncommon. I am not at all sure that it is a silk paper, however. The paper does not seem correct; it's too thin for one thing and those do not look like silk threads. You might want to spend a few bucks for a cert for this stamp.

Thanks Bart, a cert was what I should have considered first.

Just a bit of interest to add here in addition to previous comment. The strand of blue material shown in reverse photo is actually not a single strand of silk-like material. It appears, under magnification, to be a group of silk-like fibers. My USB microscope is junk, but I will attach photo anyway. Now for the question. If the paper doesn't look right, and perhaps isn't, how and what would this be embedded in the paper. I am not disputing the need for a cert, but to gain further knowledge, how does one determine what blue stands are silk and what are not. To the feel, paper is thicker than normal issue. It is very similar to my other copy with 2 individual strands in it. I will not be listing it for sale without a cert regardless.
I do realize there are more requirements than fibers alone, but really would like to know how this material would end up in paper, and what it might be if not silk. If it was silk and the paper didnt meet required thickness or type, then what? Would the cert come back and say R51c with silk fiber?

First off, it's either silk or it's not. The paper might look very different in person then it does in a scan. As for what else it might be, first issue paper is often very "dirty", with all kinds of fibers and inclusions that are not silk. They are usually described as "leftovers from the papermaking process" or something similar.