It all worked out, but not without some uncertainty
Long story quite short: It worked perfectly, and I got into the match with the ticket they gave me.
I wanted to see Liverpool v Manchester United on Oct 17, and after some research, decided this seemed like a fairly reputable re-seller of football tickets (this site, and others were used to make that decision). I ordered the ticket towards the end of September, and parted with a decent chunk of cash to make that happen. I opted for local pickup, as I was coming from Canada, and bouncing around Europe prior to the match, so could not guarantee a location for them to send me a ticket.
After purchase, I sent an email asking for information regarding the local pickup, as I was quite keen to understand the process, and make sure that they were taking care of my order after giving them a bunch of money. No response. A week later, I try again. Still no response. It's now a week before the match, and I fly out to Europe, and haven't heard from them. Needless to say I'm a bit nervous at this point. 4 days before the match, I finally receive an email saying they had my ticket, and a phone number for my local contact who I'll be picking it up from. Due to the nature of my phone plan, I waited to call the guy when I was in England, which was on Saturday (two days prior to the match). No response. Text as well, just to be sure. A few hours later, he texts me back, telling me to get in touch when my train gets me to Liverpool on Monday morning.
Cut to Monday, and I get to Liverpool. Call the contact, no response. Text him, no response. Meet up with my friends, and then eventually I get the call I was waiting for. Buddy has my ticket, but can't meet me until 6:30 or 7pm at Anfield (kick-off was 8pm). Really leaving it to the 11th hour! I was bummed, hoping to at least have the ticket before I head down to the ground, and then an hour or so later, that's exactly what happens: he calls me and says he's in the area ahead of schedule, so meets me at a coffee shop and gives me the ticket, 6 hours ahead of kick-off.
From there, no problems whatsoever; got into the ground with no fuss, and had a crazy good time at the match (regardless of United being little bitches and parking the bus against the mighty Reds). It wasn't without some tense moments, and general uncertainty, but at the end of the day, I paid for a ticket to Liverpool v Manchester United, and that's precisely what I got!