"I'll take it" is pretty clear. "PM sent" just means a message has been sent, further complicated by sellers using email accounts they don't monitor directly. A simple solution would be for the prospective buyer to be more succinct: "PM sent with questions" or "PM sent with offer" to which the seller might respond with "still available" if the offer was not accepted. Just good clear basic communication.
BTW, "I'm interested" means nothing and is really weak. If you want the thing, say so. If you have a question, ask!
+1 on posting size, gender, etc in the title. Giving basic measurements for clothing helps too: across the armpits, back length, center of collar to cuff for tops and across the waist and inseam for bottoms. Weights of course. Photos, definitely yes.
As to leaving the prices up, the sales are ultimately private transactions. If the seller wants to leave it, fine; if not, it really is no one else's business. There are so many variables that it is pretty much useless anyway.
Honesty, courtesy, and personal integrity are the keys.
The forum format is not a perfect world and the sales should be kept in perspective: this is small stuff. Breathe.
I would like to add something on Pay if Forward offers: if you accept a PIF item, follow through with a reciprocal PIF and keep the spirit of the thing going.