I would still buy but scrub or even better remove the frag plug unless a coral has encrusted over it.
I remember when set my tank up earlier this year at a national lfs ( maidenhead aquatics) i said you got an aiptasia in that tank over there incase you didnt know,manager said yeah i know,so i said you don't remove,she then says to me nope as no point keep removing them as they will always find there way in so not bought anything from them since.
Reason being aiptasia is by many a very well known pest and in every " what pest is this description" so i would thought they wouod least make effort to remove occasionally on wc day or once fortnight or whatever.
But i do also think that majority of lfs ( if not close to all ) has the majority of the common pests,and also fish dieseses as the corals/ fish mainly get imported from abroad from a exporter facility so all sharing system or systems then get shipped out to the relative countries and end up in a importer facility mixing with other corals and fish then go to lfs's then to us,and unless we paying a premium for fully pest free or un-diesesed corals/ fish then its our job to quarantine and watch or treat to make sure.
I know argument is from some that vendors shouldn't sell a coral with a pest or fish with a diesese but at prices we buy them at they couldnt do multiply dips and tank transfers and medicate,so other option is buy only from place that qt and treats everything and gives guarantee pest/ diesese free and pay a higher price so to the op if you think they actually care about their livestock and make effort to eradicate pests or give a valid reason why they dont ( i.e got bergie nudi's in tank or copperband they training to eat) then i would buy.
But if you think they neglect tanks and got attitude of cant be bothered then dont.but you could always try negotiate a discount if see pests in system and say you will or may need do more work to not get the pest yourself or can think wont try negotiate less price as want support local lfs as alot people buy more things online these days than we used to do ^_^