The vast majority of retailers on gundeals sold accessories which is legal for anyone to do.

Those that did sell guns were federally licensed retailers, gundeals did not (to my knowledge) allow for private sellers to post. All federally licensed (FFL) retailers must perform a background check (or transfer to another FFL to perform one) before transferring to a purchaser. I don't see how any of that could be considered as not "law-abiding".

All of that applies only to the US of course, but I'm not aware of other countries having deals there.

I checked /r/gundeals on nearly a daily basis and never saw anything illegal[0]. I can't find the full rules to the sub anymore [1], but I believed they banned posting gun sales that weren't through an FFL (which is actually stricter than the law).

[0] there were products that aren't legal in certain states or cities, but nothing that is illegal at the fed level.

Because it was all links to normal, legal, gun retailers with online stores, like Dick's Sporting Goods, Cabela's, Bud's Gun Shop, KY Gun Co, Palmetto State Armory, etc. (There's dozens more big ones.) All of them follow required laws in shipping only to FFL holders.

Is there any indication that these transactions were criminal? Many states allow person to person transfers of firearms, and even those that don't still allow transfers through FFLs (licensed gun stores). Unless people were selling things like unregistered NFA items, there is no indication that these sales were against the law.