I see threads like this all the time and my response? You're asking the wrong question. Trading is almost never about what's fair. It's almost always about what the other team needs. You need to be combing through the other 11 teams in your league and figure out who really needs Parker or Abdullah. Your goal is to then target players on their team(s) that maybe you couldn't necessarily get in a "fair" trade. Find a team that's had some misfortune with their WRs (think Cooper, Dez, Pryor) and have a decent amount of depth at RB. Then target someone who you think might normally be just a bit out of the price range of those guys. I've gotten more super-upgrades over the years by just playing to my opponent's weaknesses than I care to remember. This is how most of your trading should go.
The only other way to look at it is to target specific players and then figure out how much it's going to cost you to get one of those players. In that case, we'd not only need your roster, but your opponent's roster as well. And, more often than not, you're gonna have to pay a fair or even unfair price to get the guy you want.
In my experience, those are your two options for 99.9% of your trades. Trades need to make sense more than they need to be "fair", especially if all you're doing is offering up your bench fodder. Go find a team or two in your league where Parker/Abdullah would make sense and see what you can get.