Depends on the message and the sellers life I assume. I had a game listed for trade pretty recently and received a high volume of messages so I just was on the other side of this coin.
I listed what games and manufacturers I was interested in trading for, and specified I wasn't selling...lots of cash offers and trade offers for games/manufacturers I didn't have listed. I didn't want to ignore these, so had to spend time going through them all and saying thanks but no thanks.
Also got alot of messages with requests for more pictures, video, can I talk on the phone, etc. I work during the day, have a 4 year old, and we are working on a kitchen remodel currently. I tired to respond to these as best I could, but keep in mind people are asking me these questions before I even know what their offer is. Now I need to spend time doing all that before I even know if I'm going to be interested. Alot of those people will never respond again and are just picture collectors.
I list games on FB too to reach a larger audience. If you ever list games there you know they tend to get alot of interest. Now I've got messages from two places to respond to. If it's a weekend, chances are I'm not glued to my phone and out running errands, playing with the kid, etc. The people that I prioritized responding to were the ones that said "I have a game you are interested in trading for and I am interested in making a deal this weekend".
I'm not trying to frame this up as "poor me" but more that GENERALLY people are probably not ignoring you on purpose. They probably have stuff going on or just took the path of least resistance.