Yes, in AL you can buy potions, scrolls and magical items but there are limitations...
Even though Xanathar’s Guide to Everything has an entire section dedicated to buying magical items, there are additional clarifications on p. 174. (Note that though the rest of XGE Appendix A doesn't apply currently to Adventurers League, the statement below is accurate in referring to current practices in AL adventures.):
Characters can use their monetary treasure to purchase anything from
the equipment lists in chapter 5 of the Player’s Handbook. In
addition. the Adventurers League allows characters to purchase potions
and spell scrolls...A spell scroll can be purchased only by a
character who is capable of casting the spell in question.
Spending downtime is one limitation...
For the nuance in your question about “possibly using downtime” - there are in-adventure opportunities in Adventurer’s League to buy magical items but note that there are often limitations including spending downtime.
For example, in Tomb of Annihilation downtown costs and magical items sold are cited in the TOA Primer:
...While the merchant princes are happy to sell the
items over which they hold monopoly, doing so can
be time consuming. Characters wishing to purchase
goods from the merchant princes must spend 10
downtime days arranging a meeting, negotiating
prices, and coordinating for impartial intermediaries
to accept, transfer gold during the transaction. At the
end of the ten days, you may make one purchase—
either a single permanent magic item, or up to their
limit in other items (see, below).
Potions and scrolls can be bought in this AL adventure:
...In Tomb of Annihilation, the merchant Wakanga in Port Nyanzaru sells potions (common & uncommon)
and scrolls (1st, 2nd and 3rd level)
Magic items can be bought in this AL adventure:
...In Tomb of Annihilation: Ekene-Afa. This Merchant Prince has a small
selection of magic items (p.25) readily available for a
modest price. These items can’t be traded and the
permanent magic items increases the character’s
magic item count by one. The character may
purchase a single item or no more than 10 pieces of
+1 ammunition in a single transaction before she
refuses the character further service.
Outside of AL adventures, using downtime, you can only buy non-magical and magical items listed in the Player’s Handbook. Currently, the only potion in the PHB is a Potion of Healing and there are no scrolls.
Magic items also cannot be sold by PCs.
What can be sold is also limited. According to the AL documentation:
The first rule is relatively simple. There will be some rare
opportunities to buy magic items with gold, but never to sell them.
Adding more magic items into the world is okay; characters that get
more items simply won't get items in contention after an adventure
session. We just have to preserve a character's total permanent magic
item count in order keep things balanced.
Non-AL games do have guidelines for purchases.
This is in contrast to non-AL games which can allow downtime magical item purchases per Xanathar’s guide:
Buying a Magic Item
Purchasing a magic item requires time and money to
seek out and contact people willing to sell items. Even then, there is
no guarantee a seller will have the items a character desires.
Resources. Finding magic items to purchase requires at least one
work week of effort and 100 gp in expenses. Spending more time and
money increases your chance of finding a high-quality item.
Additional details and tables are provided for rolling on the outcome of the search.
