I found this months ago, it's a Userscript (Javascript) for Firefox's G r e a s e M o n k e y/Scriptish add-on.

Script Summary: Warns viewer of potential scammers on for mpgh.net/forum - Firefox
Code:
// ==UserScript==
//  @NaMe             MPGH Scripts - Blacklist User Warning - Firefox
//  @Namespace          xerotic/MPGHscammersff
//  @Description      Warns viewer of potential scammers on for mpgh.net/forum - Firefox
//  @include           *mpgh.net/forum/showthread.php*
//  @include           *mpgh.net/forum/member.php*
//  @include           *mpgh.net/forum/forumdisplay.php*
//  @Version           1.0.0
// ==/UserScript==

scamList = 0;
check = false;

function genList(){
  var thebiglist = GM_xmlhttpRequest({
    synchronous: true,
    method: "GET",
    url: "http://dl.*******.com/u/64258819/mpghblacklist.txt",
    onload: function(response) {
      processTheRest(response);
    }
  });

}

genList();

function processTheRest(response) {
    scamList = response.responseText;
    scamList = scamList.toString();

    var scamArray = scamList.split(/\r\n/gim);

    for(var i = 0; i < scamArray.length; i++) {
        tempScam = scamArray[i].split(/\|/);
        tempTID = tempScam[1].split(/\:/);
        for(var y = 0; y < tempTID.length; y++) {
            tempScam[y + 1] = tempTID[y];
        }
        scamArray[i] = tempScam;
    }

    if(window.location.href.toString().indexOf("showthread.php")!=-1) {
        var userinfo = document.getElementsByClassName("userinfo");
        for(z in userinfo) {
            var morespecific = userinfo[z].getElementsByClassName("largetext")[0];
            for(y in scamArray) {
                if(morespecific.innerHTML.indexOf('?'+scamArray[y][0]+'"')!=-1) {
                    
                    newHTML = "<a href=\"javascript:void(0);\" onclick=\"if(document.getElementById('idscam" + z + "').style.display == 'none') {document.getElementById('idscam" + z + "').style.display='block'; } else { document.getElementById('idscam" + z + "').style.display='none'; }\"><b style='color:red'>[Marketplace] Blacklist User</b></a><div class=\"author smalltext\" id=\"idscam" + z + "\" style=\"position:absolute;display:none;padding:10px;background-color:#333333;border:5px ridge #4F3A6B;\"><span style=\"text-decoration:underline;\">Blacklist Database:</span><br />";
                    for(x in scamArray[y]) {
                        if(x != 0) {
                            newHTML = newHTML + "<a href=\"http://www.mpgh.net/forum/showthread.php?" + scamArray[y][x] + "\" target=\"_blank\">http://www.mpgh.net/forum/showthread.php?" + scamArray[y][x] + "</a><br />";
                        }
                    }
                    newHTML = newHTML + "</div> - " + morespecific.innerHTML;
                    morespecific.innerHTML = newHTML;
                }
            }
        }
    } else if(window.location.href.toString().indexOf("member.php")!=-1) {
        var z = "";
        var morespecific = document.getElementsByClassName("largetext")[0];
        for(y in scamArray) {
            if(document.body.innerHTML.indexOf('?'+scamArray[y][0]+'"')!=-1) {
                newHTML = "<a href=\"javascript:void(0);\" onclick=\"if(document.getElementById('idscam" + z + "').style.display == 'none') {document.getElementById('idscam" + z + "').style.display='block'; } else { document.getElementById('idscam" + z + "').style.display='none'; }\"><b style='color:red'>[Marketplace] Blacklist User</b></a><div class=\"author smalltext\" id=\"idscam" + z + "\" style=\"position:absolute;display:none;padding:10px;background-color:#333333;border:5px ridge #4F3A6B;\"><span style=\"text-decoration:underline;\">Blacklist Database:</span><br />";
                for(x in scamArray[y]) {
                    if(x != 0) {
                        newHTML = newHTML + "<a href=\"http://www.mpgh.net/forum/showthread.php?" + scamArray[y][x] + "\" target=\"_blank\">http://www.mpgh.net/forum/showthread.php?" + scamArray[y][x] + "</a><br />";
                    }
                }
                newHTML = newHTML + "</div> - " + morespecific.innerHTML;
                morespecific.innerHTML = newHTML;
            }
        }
    }
    else {
        var thread_creator = document.getElementsByClassName("author smalltext");
        for(x in thread_creator) {
            for(y in scamArray) {
                if(thread_creator[x].innerHTML.indexOf('?'+scamArray[y][0]+'"')!=-1) {
                    newHTML = "<a href=\"javascript:void(0);\" onclick=\"if(document.getElementById('idscam" + x + "').style.display == 'none') {document.getElementById('idscam" + x + "').style.display='block'; } else { document.getElementById('idscam" + x + "').style.display='none'; }\"><b style='color:red'>[Marketplace] Blacklist User</b></a><div id=\"idscam" + x + "\" style=\"position:absolute;display:none;padding:10px;background-color:#333333;border:5px ridge #4F3A6B;\"><span style=\"text-decoration:underline;\">Blacklist Database:</span><br />";
                    for(z in scamArray[y]) {
                        if(z != 0) {
                            newHTML = newHTML + "<a href=\"http://www.mpgh.net/forum/showthread.php?" + scamArray[y][z] + "\" target=\"_blank\">http://www.mpgh.net/forum/showthread.php?" + scamArray[y][z] + "</a><br />";
                        }
                    }
                    newHTML = newHTML + "</div> - " + thread_creator[x].innerHTML;
                    thread_creator[x].innerHTML = newHTML;
                }
            }
        }
    }
}


But, there's a problem.

Code:
    method: "GET",
     url: "http://dl.*******.com/u/64258819/mpghblacklist.txt",
     onload: function(response)
The ******* was temporarily deleted the first time I checked, and it still is. Even if it wasn't, this list must be very outdated.

All I'm asking for is a community-sided list of reported users and users who may be scammers. If this was worked as a community-sided project -- made by the users, for the users --, then we could possibly get something like this up and running again.
I would've done it myself, but I'm not as experienced as the users here. It would be nice if multiple users here created their very own list of known scammers, then have them all merged into list.

Maybe one day, this could be a big cumminity mission~