I'm not familiar with the site; but the writer may well be of BrE origin.
(In BrE, you could use "sells", "should sell", or the subjunctive "sell". The last option has become more popular in the last two decades, in BrE, in contexts where "should sell" would once have been predominant.)
MrP
Thank you soo much. That was really a satisfactory explanation.[Y]
The subjunctive mood forms:
should sell and sell are the formal ones. Sells is informal.
It says "Mark suggests that he sells/should sell his house" is correct.
(In BrE, you could use "sells", "should sell", or the subjunctive "sell". The last option has become more popular in the last two decades, in BrE, in contexts where "should sell" would once have been predominant.)
MrP
should sell
and
sell
are the formal ones.
Sells is informal.
for example order , mandate
suggest he be the King
order he be the King
etc