Arsenal are reportedly entertaining the idea of selling one of their longest-serving players, Hector Bellerin, with reports emerging that Paris St-Germain and Barcelona are interested parties.The news has undoubtedly upset plenty of Arsenal fans, who would loathe seeing a beloved player in another shirt, albeit in a different league. Of course, there are those who will consider the deal good business, but there are plenty of arguments to be made against selling Bellerin.Here are 5 reasons why Arsenal would be making a terrible mistake by letting their Spanish fullback leave:#5 Bellerin is still an amazing playerIt's fair to say Hector Bellerin has not had the best form over the last couple of seasons. The Spaniard has looked a far cry from the player who broke out with such aplomb under Arsene Wenger that Mathieu Debuchy's Arsenal career evaporated in the blink of an eye.However, there have been mitigating circumstances - Bellerin has had to cope with a major ACL injury and was straight away thrust back into action as Arsenal (and Bellerin) struggled under Unai Emery's less than obvious instructions. However, the Spaniard has looked much like his old self following the COVID break, as he seems more in tune with the demands placed on him by Mikel Arteta.This is still a player coveted by top clubs and there is not much doubt about his quality. Bellerin still has glimpses of explosive speed and his positional sense has only improved with experience. These are qualities that Arteta's Arsenal could do with, and a reason why it would be foolish to let him go.Hector Bellerin 🇪🇸 || Fan favorite for a reason ❤️ pic.twitter.com/nkuUAerfdY— RP (@TheArseSA) September 4, 2020#4 The numbers do not make senseIf the rumours are correct, Paris St-Germain are willing to offer Arsenal an eventual fee of €35 million, in a deal structured as a loan with an obligation to buy.Putting aside the fact that Bellerin is worth much more than €35 million, the entire point of Arsenal being open to a sale is to raise money now, so as to fund purchases of Houssem Aouar and Thomas Partey. There would be no benefit to allowing Bellerin to leave on loan, no matter the price PSG offer.Barcelona, meanwhile, have reportedly valued Bellerin at €20 to €25 million, a fee perhaps keeping in mind Bellerin's La Masia past and Catalan roots. Given Nelson Semedo's disastrous performance against Bayern Munich, Arsenal would be well within their rights to ask for double the fee for a first-team player who's contracted with them for quite a while.Aubameyang’s inclusion in the PFA Premier League Team of the Year means he is the first Arsenal representative since Héctor Bellerín in 2015/16. #afc pic.twitter.com/6jYYQrA5JW— afcstuff (@afcstuff) September 9, 2020